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Is there an OpenRasta like REST framework for Java?

The circumstances force me to start writing a java based server and I am looking for a REST framework for Java. I really like the way OpenRasta works - a REST framework for .NET, so my question - does anyone know a REST framework for Java which works similarly to OpenRasta?
Thanks.
EDIT
Unfortunately, I am not in a position to be an early adaptor, so well established frameworks have preference over emerging ones.

You want to use one of the implementations of the JAX-RS specification, one of the main ones being Jersey. The specification and it's implementations have been around for a long time and are tried and tested (as opposed to Play! which you don't seem to like due to it being a newer framework).
I used OpenRasta and then worked on Java projects using JAX-RS. It will feel very familiar and you will get the benefits of POJOs like you get from OpenRasta's use of POCOs. I feel it's a great choice for creating RESTful services. In fact, I would probably chose it over OpenRasta now but this is more due the benefits of working within a Unix environment than the frameworks themselves.

The play framework is a popular new choice for Scala and the JVM:
http://www.playframework.org/documentation/1.2/routes

One option is Play!, which is basically framework based off Ruby on Rails.
Second option you might be interested in is Lift, which also has really cool features like comet, AJAXy snippets, etc. But it is a little bit different than Play!.

As part of this project, I would recommend Restlet which is the first and most comprehensive REST framework for Java. It works on both client and server side and is indeed resource centric, not just a MVC framework adapted to REST.

WHich is the best path or flow to learn java from web site perspective

I have just started learning java and i have about 8 months time. My main aim is to build web application using java. But as there are many java technologies/frameworks available i am conufused how should i go
IDE i am using is eclipse
The path which i am thinking is below
1)Learn Core Java
2)Project: Build Java Invoicing System with JDBC or mysql to test java knowledge
3)Learn Servlets / JSP to
4)Build small site like simple logon/memberarea/catalogue using servlets/jsp
Now guys I really don't know where the following things stand in my path and which things they will help
1)Java EE
2)Spring or spring MVC, what's the difference?
3)Hibernate
4)Wicket
5)Struts
I really don't know how should i learn those and which things to choose from.
what should be my final project which can be build using those because if i see from my current stage even website can be build using servlets/jsp. then why do i need those other technologies
Can anyone give an example of a project which can be build using new technologies and not using jsp/servlet, so that I can get an idea why we need those?

Lot's of stuff to learn in here so I would take it a bit at a time. I'm going to assume you are relatively comfortable with the Java language and core APIs.
First learn about JDBC and databases. Get a basic functional JDBC example app working (not visual just some a vanilla Java app) that does read/insert/update/delete of some records in a table. This will give you the basis for building whatever app you choose. Almost all interesting applications involve some sort of persistence/database.
Learn the main JDBC APIs (Connection, PreparedStatement, ResultSet) and write some code that uses these to get comfortable.
Learn about how transactions work in databases if you aren't familiar already and how JDBC uses them (Connection.commit and rollback)
Next you could look into the spring framework. This has a several useful features that come in handy when building these applications. The main ones to get to know the dependency injection functionality and the JDBC libraries it has. The DI stuff is a bit of a "duh"? if you haven't used it before but it is very useful/powerful especially for medium size + apps. The JDBC libraries help with the cruft of dealing with the raw JDBC APIs and make your code less error-prone.
Once you've got some basic JDBC and Spring stuff worked out you can then start bringing in a web framework.
This is more difficult because there are so many. In the interest of getting something up and running quickly I would probably go with Wicket. It runs inside a servlet container like Jetty, easy to get up and running with a basic web page or two and relatively easy to evolve of the project over time without going down too many blind alleys. I've found that frameworks like JSF (the Java EE default web/view framework) takes a lot of up front time investment to get going on. Most of these frameworks, including Wicket, can leverage the spring stuff you've got above.
Avoid JSP, it's unpleasent technology and outdated by virtually everything else.
To answer some of your other points.
Hibernate is a tool for mapping an object model to a relational model. In it's most basic form you define a Java class for each DB table with the class having one Java property (variable+getter+setter) for each DB column. You can also add references from one class to another to model database relations. It is good but it takes a bit of getting used to.
Spring I've explained partly above. Spring is much bigger than just DI and some JDBC libraries. It also has transaction management libs and bunch of other stuff. Spring MVC is a web framework which lives under the spring umbrella and makes use of a lot of the spring libraries internally. I've never used it so can't offer an opinion on it.
Java EE is an umbrella term for a large set of "enterprise" specifications/libraries. JSF is a web view framework that is part of Java EE.
That's my very high level advice. To summarise I would learn basic DB/JDBC then some spring stuff and then start building a basic web app using what you've learned already.
Hope that helps. There's plenty of info out there on the web on all these topics and you can always ask questions here on more specific parts if you need help. Good luck and enjoy!
EDIT (to address comment):
All of the above is my recommendation/opinion on how to approach learning Java web development.
In summary
Learning JDBC/transactions/databases is a must
Spring is optional but I strongly recommend. Spring is a big library but you can pick and choose the bits you like/need.
Web frameworks the choice is really yours. From what I have tried (JSF/JSP/Wicket) Wicket is the easiest to get going with so you don't spend too much time frustrated with the getting the initial setup. JSP is awful, it's very easy to make a mess with it. JSF is powerful but heavy and probably more suited to very enterprisy projects than a first web app.
Hibernate is optional but can make life easier simply because you can work with Java objects when you do DB queries/updates rather than writing tedious insert/update statements. Hibernate isn't the only tool like this but it's the one I've used most and does the job well. Just don't go too overboard with the "clever" hibernate features initially.
You could easily use Spring MVC instead of Wicket. That may be a perfectly good choice, I've not used Spring MVC myself so can't comment. I'm sure it will integrate well with spring stuff tho so that would certainly be a positive factor.
Do some more research if you're not sure, there's no end of people happy to give their opinion! But really you just have to dive in and try something.

You are correct. If all your applications are served by servlets, JSPs, and JDBC, then maybe you don't need to learn anything else.
I commend you for learning the fundamentals first before diving into a thicket of frameworks that you don't understand. That's a good thing.
But if you reach the point where you have those down cold, maybe looking at these other technologies can help you improve your game.
1)Java EE
You are learning (part) of Java EE when you use servlets, JSPs, and JDBC. They're a subset of the full Java EE machinery - EJBs, JMS, JNDI, etc.
2)Spring or spring MVC whats
difference
Spring is an alternative framework developed by Rod Johnson and Springsource, now part of VMWare, that is based on dependency injection, aspect-oriented programming, and framework modules. Spring web MVC is one module in the Spring framework, based on servlets, that acts as the front end for web applications.
3)Hibernate
An object-relational mapping technology (ORM), built on top of SQL and JDBC, that lets you map objects to tables. It has its own object-based query language.
4)Wicket
Another web MVC alternative to Spring MVC, Struts, JSF, etc.
5)Struts
The first web MVC framework. It's gone through versions 1.0 and 2.0, and has now been supplated by Java Server Faces. It's still used, though. Like all other web MVC frameworks in Java, it's based on servlets and JSPs.

From a Java perspective it might be best to start looking at Java EE. All the others you mentioned are alternative technologies, which might be useful to you if you decide that what the standard framework offers isn't your cup of tea. But for that to decide, it's never a bad idea to at least know what the standard framework is about.
Truth be told, before 2006 the standard framework had a bad reputation and some of the sentiments that form the basis for recommending alternative technologies are still based on that. Starting with version 5, Java EE got dramatically better. The latest version, Java EE 6 is arguably one of the best Java frameworks that's out there. Of course, the best is a highly subjective term and naturally it won't be the best one for each and every person out there.
At any length, Java EE 6 is a very complete framework that allows a large range of application to be written without depending on any additional library. Being the standard framework, most other frameworks at least depend on some parts of it.
Most typically EJB, CDI and JSF are replaced by alternative technologies. E.g. the core Spring container replaces EJB and CDI and Spring MVC replaces JSF. A full Spring stack typically still uses JPA, JTA, JMS and Servlet from Java EE. Wicket on its turn only replaces JSF, or when used with a Spring stack replaces Spring MVC.
Hibernate is a special case. It doesn't replace anything from Java EE, but is instead often used as an implementation for one of the key APIs of Java EE: JPA. The original creator of Hibernate, Gavin King, is one of the prime supporters of the Java EE framework and is in fact the spec lead of one of the most important parts of modern Java EE: CDI.
Wicket by itself is a very nice web framework, but in practice it isn't used as often as JSF. I won't go into the discussion whether Wicket or JSF is 'better' (this is mostly a religious battle anyway), but due to the popularity of JSF there are simply more people experienced with it and there is a large community offering many things for JSF like component libraries and extensions.
Struts has historically been completely replaced by JSF. The original creator or Struts, Craig Mcclanahan, was the one who started JSF as the successor of Struts. Nevertheless, Struts was once the absolute de-facto standard for web frameworks in Java. It's not often advised to be used for new projects, but till this day it's still used in a huge number of existing applications. So even though it's not really 'hot' to learn Struts anno 2011 knowledge of it may still be very practical for when you have to maintain existing applications.
ps
See this answer for a general description about Java EE: Frameworks for Layering reusable Architectures

I'm sure that nobody seriously can tell you the best way, because this would mean that (s)he would have tried all. Why do you want to learn a Java web application framework? Just to learn it to be prepared for the (next) job? In that case it is likely that you've learned the wrong one.
The next question would be what you want to do with the web application framework. Do you want to make a website a little bit active, e.g. make each page look similar (corporate design), auto-generate menus from an internal structure, or do you want to write a real web application, e.g. with database access.
I'd first define the goal what you want to achieve. IMHO a good way to motivate you to make it right is to take a small project which helps you to solve a small problem.
Then I would start with Java Server Pages (JSP) and servlets to understand the basic concepts. Then you can try to do the same with a few well-known web application frameworks, e.g. Wicket.

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I am a .NET developer (and have been for a while now). I work for an organization that was just recently acquired by a larger company whose primary development language is Java. There are a few .NET developers, but the ratio of .NET to Java has decreased substantially now that the teams have merged.
That being said, I've decided it would be best for me to start Java development. However, most of the books I've seen so far for "learning Java" all take a very basic approach (what is a class, OOP principles, etc etc). I am comfortable with this part of development and don't need a primer (unless there are differences so profound that someone recommends the fundamentals from a Java perspective...).
Anyway, I'm looking for a book recommendation for Java development from a software developer's perspective that discusses today's techniques. For example, MVC architecture, application best practices (I am a web developer, this includes web services), is it worthwhile to work with JSPs or consider Ruby instead, etc etc.
A HUGE bonus would be "learning through doing". Something like Murachs, where I can step through a project from start to finish, and is light enough on fundamentals that I don't get bored. I'm hoping to walk away with enough basic knowledge to volunteer for some internal projects and grow from there.
I'm sorry if my question is needlessly broad, but I'm struggling to find a starting point aside from my Eclipse installation (I am doing this on Ubuntu, deliberately avoiding Windows).
Thanks for any direction or insight you can offer.
EDIT - After discussing with a co-worker, and reading Bert's great suggestion (all of them have been excellent, thank you all very much), it turns out the main focus is on EE, and Glassfish. They use NetBeans for development, since it is tightly bound to Glassfish.
This doesn't mean much to me, except that I think the parallel drawn is IIS/Web apps to Win32 apps. But perhaps it will help clarify some of the more open-ended questions in my OP.

Effective Java By Josh Bloch
It may not be "learning by doing" but it gets into the details of how to use the Java language effectively.
I would then complement it with Java Puzzlers by Josh Bloch and Neal Gafter

My best bet is to learn Spring based on your requirements:
MVC architecture, application best
practices (I am a web developer, this
includes web services), is it
worthwhile to work with JSPs or
consider Ruby instead, etc etc.
You can start by visiting SpringSource at http://www.springsource.org/

There are a few points I would like to make to you:
The basic Java language is rather small and C-ish. To my understanding C# and Java works almost identically here.
The Java runtime library is VERY, VERY BIG, and rather unlike C#. Very few people know all of the Java 6 runtime library well.
There are several IDE's in common use for Java. You will want to learn the one used by the rest of the team.
I would suggest having a brisk walk through the Oracle Java Tutorial, just to get the hang of the spirit and do some of the exercises, and then look at the common "Java for C# programmer" cheat sheets on the internet. Then do a lot of code, and then read Effective Java.
(For those who think they know Java 6 runtime well, please consider if you are familiar with RMI, the Rhino Javascript engine, and XPath in the XSLT implementation and all the rest)

Please look at the following thread:
https://stackoverflow.com/questions/75102/best-java-book-you-have-read-so-far

Ruby makes no sense unless you're writing JRuby on the JVM.
If you're writing web apps, you'll need to know JSPs written using JSTL (JSP standard tag library), servlets, HTML, CSS, and JavaScript. Download Tomcat from Apache - it's a servlet/JSP engine that will let you deploy web apps locally for learning. It has an HTTP listener built in, so you don't need IIS or Apache.
You'll need to know how to create and deploy a web archive (WAR) file. That's a ZIP file that contains the standard format for a web application.
I'd have a look at the Tomcat "first web app" docs to get a feel for it.
Once you have that, come back and talk about frameworks. (When you do, I'll recommend Spring.)

Here are some references I like. I'm sure there are better, but there are the ones I have read and can vouch for:
Java in a Nutshell is a classic summary of the core language. If you need more, there are books like Java for Programmers for programmers transitioning to Java (I have not read it).
Sun's Core Servlets and JavaServer Pages was a good book for Java servlets and JSPs. But consider that a lot of web tech is built on top of this (e.g. Spring), so this may be giving you foundation but not direct knowledge of the specific framework you will be using. Also a lot of web tech these days are client-side/JavaScript/AJAX-based, e.g. jQuery.
As for MVC, consider focusing on MVP instead. I been playing around with Google Web Toolkit to leverage my Java background to create client-side apps, but it is also a good way to get some idea of MVP - read Large scale application development and MVP Part I and Part II, and the GWT MVP Development with Activities and Places. However, GWT keeps evolving.
My last suggestion is that you narrow your focus - try to figure out what frameworks are popular at work and decide if you want to focus on a tier: front (e.g. HTML/CSS/jQuery), middle (e.g. Spring), or back-end (e.g. Hibernate). Once you figure this out, then post a more specific question(s) (e.g. what are the best resources for learning XYZ).

Just from my experiences with java (limited but I took a Web Dev course teaching JSP's) Any decent technology for it is usually 3rd party. However, from what I can tell when doing web dev I would use Netbeans, Netbeans has Tomcat built in especially in Ubuntu for easy debugging.

More on similar lines you can find answers from this post
https://stackoverflow.com/questions/3820437/life-after-head-first-java/3820449#3820449

The Java landscape is quite wide as you probably have already figured out.
There are hundreds of frameworks and tools that can be used for basically doing the same thing. To get an idea, you can take a look at this presentation written by Matt Raible, where he compares a number of Java web frameworks.
As someone else suggested, you should definitively take a look at the Spring Framework. It is widely used in the enterprise world. There are several good books about Spring.
If you want to get the basics of Spring MVC in a "tutorialized" style, you can take a look at this book. It doesn't cover the latest Spring version (3.0) and it is not a "perfect" book, but it should allow you to get the basics while running some examples.
Also, there are literally thousand of Spring-MVC tutorials on the web.
For instance, you can take a look at:
http://www.adobocode.com/spring/a-spring-web-mvc-tutorial
RESTful services are also quite popular these days. I have been working with the Resteasy framework from JBoss and I found it very easy to setup and work with. This book explains the REST architecture and uses Resteasy as framework for the examples.
I also recommended "Head First Design Patterns" from O'Reilly. It covers the most important patterns using Java and it will help you to see how the language can be used for patterns you may be already familiar with.
One more suggestion: you may considering "tuning in" on Java TV. It's a great resource with hundreds of Java tutorial videos. It covers a wide spectrum of technologies so you should be able to find something for you.

Which Java technology should I use, if I want to build a website with thousands of users?

Sorry for asking this question, but I searched all Java-related questions, but I got more confused. I am still not clear what should I start with.
My main thing is building websites in Java, because someone told me that there are some machine learning or AI libraries available in Java which I can use in Java. So I decided to use that as it can reduce my work.
Now I don't know Java at all. Some people say the following are used to build websites, like:
Servlets alone can build a website
JSP alone can also build a website
Struts
Spring Framework with Hibernate
Seam
Java EE also for websites
I am confused. Where should I start from? Where does core Java fits in here?
I was thinking of perhaps learning Python as I know I have to learn Python only, not its variations.
So please guide me to one thing which can solve my basic purpose of using ready-made libraries of AI.
If I can do that with JSP then I will start with that.
But if I need to learn all of them, then it’s better if can start learning Python.
I have five months to finish the website.
I really don't know why there are many branches of Java for doing one thing.

To correct some misconceptions of yours:
servlets and JSP are Java EE
Seam and Struts can be used together with Hibernate as well
Spring MVC is a web framework. Spring as a dependency injection framework can be used together with any other framework.
there are many more options, like JSF, Tapestry, Click, Play, etc.
It depends on what will the site will be doing, but you can choose any.

Experience has shown that building complex HTML forms (especially validation) and Ajax functionality is hard to get right. You will most likely appreciate having a library to assist you with that. The same thing with database access.
Everything builds on servlets. Learn those in order to understand the libraries you will use.
I would suggest learning JavaServer Faces (JSF) with facelets. The easiest, while being powerful, is JSF 2 in Java EE 6. The easiest way to get started is to use NetBeans with GlassFish 3.

For static web pages, HTML alone is enough and if you want to make it more user-friendly and interactive, use JavaScript.
There are lot of frameworks you can use directly in JavaScript, like jQuery. Now if you want to make some more functionalities, like database interaction, then you will need either PHP, Java (JSP and Servlets) or .NET (there exist some more, but I’m aware of three only). Each of them have their library to interact with database.
Since you are interested in Java, go through Servlets, HTML, JavaScript, and JSP. Once this is done, you can look into JSF and EJB beans. So if you have knowledge of Java this will not take too much time, because the syntax is the same; the only difference is the concept.
So you will have a frontend (HTML, JS, and JSP) which will interact with the backend (Servlets and the database) which will be deployed in a Web container. To deploy, you can use an one of the available application servers, like Apache Tomcat, GlassFish, or WebSphere. To develop your application, you can use Eclipse as the IDE, which is one of the best IDEs I have used.
So decide your architecture and design, break it into components and start writing small applications first. Once you are experienced in small application, start writing an application where there are more things you have to care of, like load on database, availability, security, etc. For that, you can refer a very famous question on Stack Overflow, What technical details should a programmer of a web application consider before making the site public?.

The closest to core Java is servlets. No extra syntax, very minimal library, but no included functionality.

For building websites, the main thing is HTML and HTTP.
All technologies (including Enterprise Java or Server side Java) do the same—to understand an HTTP request and respond back with HTML. All that you mentioned do the same thing with a different level of sophistication.
Start with Wikipedia.org to know what technologies do what. If you are completely new to Java, go through the Sun tutorials, do sample programs, and get comfortable with Java packages, JVM and classes. (This might take a week or two).
To build websites using Java, you'll have to learn Java Servlets—everything else (JSP, Struts, JSF, and Spring(!)) are built on it. Get Head First books on Java and Servlets; they will be useful.

You can always build the page in Ruby or something that will give you a website very fast. For the AI stuff that analyzes the bought items for the recommendations, use something unrelated to the page that only takes the database to work with.
I would not mix the whole web page stuff with the AI computing. You can do the AI things that will take a long time and maybe should done on a different machine to have your webpage also responsive with C++ or Java in normal applications.
Choosing your front end technique because of later usage is a very dangerous thing. Using the whole Java EE concept and learning it from scratch will take you much longer than have a quick start with your page if you do it in Ruby or PHP.
You can then build the first version of the page and generate a user base. Start working on the AI stuff once you got some data with other technologies.
If your page has performance problems because your technology does not scale, you can always begin switching parts of the stuff out with a faster technology or maybe simply a bigger machine. Get your page running before you build it bulletproof for the thousand users you are expecting.

If you’re trying to build a website from scratch, with no prior knowledge, your best bet is to go with frameworks that provide a complete stack for web development.
If you aren't dead set on Java, you could probably start learning Ruby on Rails, which would be easier to start with (if so, go buy a Ruby on Rails tutorial which goes through all the necessary things for modern web development, from MVC to version control).
For Java, try:
Play Framework is very similar to Ruby on Rails. It will get you started very quickly, and hopefully (if you follow conventions) handle all of your peripheral needs.
Spring Roo is a very-fast-to-work-with framework, but it might get hard to maintain and extend later on.
These two are full web frameworks, that is, they handle database, MVC (presentation layer), configuration setting, etc... (I'll explain shortly). And could even ease your deployment process.
If you're going to go through any other route (using several frameworks together), you are probably not going to finish what you are doing in five months.
To understand why, and to clear some things you wrote in your question, you need to understand that modern web development (and enterprise development as a whole) is comprised of several different technologies:
Presentation: the term web frameworks has been more or less taken to mean frameworks that deal with this side of web development, i.e., how to create a web page (an HTML page). Most of the frameworks offer an MVC like approach (which you can read about elsewhere), and this is what most of the frameworks/technologies you gave sit in—JSP and Struts (which uses JSP) are examples of web frameworks. Servlets is a lower-level API that standardizes how to answer HTTP requests (which JSP and Struts sit on top of).
Persistence: being able to show a webpage doesn't mean squat these days if you can't save data to a database, and that is what the persistence frameworks do (they ease the way you can save data to the database). Hibernate is a persistence framework.
For Java there are also dependency injection (DI), which is what Spring DI is. It's tough to explain DI to people not familiar with the troubles of Java, so I won't try to explain it, but it's a very important part of modern Java development.
apart from these core technologies, there are also many items that you'll need to learn to actually deploy the application (unless your using services like Heroku or Google App Engine, which should simplify things), and troubleshoot it later (understanding how concurrency and HTTP works).
To address the other technologies you brought up:
I've never really touched Seam, so I'm not sure, but it looks like a collection of other frameworks (mainly EJB 3, which itself is a collection of APIs, and JSF).
Spring denotes a popular open source group (the SpringSource group) that provides various technologies, but most likely when you hear about it they refer to two things Spring's dependency injection (DI) and Spring's MVC (which is a web framework).
Java EE is merely a collection of high level APIs which other technologies implement (Hibernate implements JPA which is part of Java EE for instance).

Playframework and Django

I have worked with Django before and have recently seen the Play framework.
Is this the Java community's answer to Django? Any experiences with it? Any performance comparisons with other Java web frameworks?
Edit:Almost similar to this question, the responses, unfortunately don't say much about the framework.

Play! is a breath of fresh air into Java and bypasses all the Enterprise cruft that has evolved over the years. Even the namespace is just play not com.playframework. It is supposed to be an answer to Rails, Django etc and is MVC based. It is needed for Java to stay relevant in all but deep entrenched enterprise shops.
Play! reduces the overabstraction and painful configuration of old Java. It is a complete stack it does not rely or play to the old Servlet/EJB methodology like Restlet tried to do (making REST easier in Servlets). Play! is a great REST based Java framework that is a valid contender to other platforms MVC frameworks.
It is very RESTful and it is easy to bind a parameter to a java method. They have also made JPA much easier to use through their play namespace.
play.db.jpa.Model
public void messages(int page) {
User connectedUser = User.find("byEmail", connected());
List<Message> messages = Message.find(
"user = ? and read = false order by date desc",
connectedUser
).from(page * 10).fetch(10);
render(connectedUser, messages);
}
Python is used for scripting instead of builds with Maven which might save a few lives.
I haven't been this excited about a Java framework since Red5 or Restlet. A bonus is they have easy ways to get your app up on Google AppEngine as well using the Java version of GAE.

I have been using Play! now for a few months and in fact have come to love the framework. I struggled with Rails and Django a bit, mostly because I am really not a fan of dynamically-typed languages; however, there was never a really good web development framework for Java to compete with these. In terms of productivity, Rails and Django were the leaders for the MVC arms race that was going on. Play! is awesome, it's concise, scalable, powerful, and it has a great community that is growing all the time. If you're still really into using a language like a Python or Ruby, you can use Play! with Scala too. I am really trying to get into Scala right now because I think it has a great future and it's a lot of fun to use. Anyway, I would recommend giving it a try!

The Play! framework is a really good piece of software, and that the JavaEE bloated environment should be inspired from.
I moved from Java -> Django because of the fast cycle "modify file" / "reload browser", and the Play! framework makes me came back to my favorite Java language.
It could also be compared in some terms to what Grails and in general dynamic languages in Java (Groovy is used in Play!) are trying to import: simplicity, speed and reliability.

I am also a Django user. I've just visited the Play framework and skim thorugh its documentation. It has the simplistic design Django has been known of. It even has app engine support built-in. I'm sure many java developers will support it, and it only need some time to see cool plugins from the community.

We recently started using Play for building a webservice for various mobile applications. I come from a Java environment. I can tell you that the learning curve isn't all that steep--literally in an hour I had the webservice running with basic API already. One week later we were on Amazon Web Services. I definitely see a future for Play as it simplifies web development for Java developers.
Couple of things that I noticed however (asset versioning, etc.) still are not built into the framework, but i'm sure they'll be there in time. I would say it is definitely worth a shot using Play.

I come from a very strong java background. So my answer here could be a little biased.
Play finally brings to the java community what django has been for all these years in the python community, but just a way better.
Play is built on the jvm therefore inheritance all the goodies from a solid platform that has been proving over the years to be the most reliable and scalable one that allows to write and run applications at scale.
I want to say that I did try django. Its popularity among the web community made me curios and I wanted to give it a try.
Strangely I did not find it as easy to use as I had expected.
So many configurations. Too may libraries doing the same thing and often not play very well with each other. A way too much magic.
Furthermore, not having type safety makes very hard to manage and maintain
web application at big scale.
Don't get me wrong, I am pretty sure that people managed to do it, but in my experience I still find java/scala best suited for this, especially when you share the code base with a lot of other developers.
IDE support for Java it is unbeatable. If you implement TDD you will find yourself refactoring code and moving things around on the daily basis. And java IDEs give you all this power. With type safety and more.
My take away is that as long as you find yourself writing a simple CRUD application/prototype/toy without even thinking too much of advanced features and big scale then you can probably find some advantages on using python/django. Otherwise the whole java ecosystem wins hands down. And play is the cherry on top.