I've been using Play! since Dreamforce 11 when Heroku announced support for it and I have to say that I'm addicted to it. If you love Java (and who doesn't) but hate developing Java applications with all of the crap that goes along with it (Maven, XML config files, deployments, etc.) then the Play! framework my just be your savior.
I did a small demo with salesforce.com integration with Heroku and Play! if you would like some sample code or to run it for yourself.
Here's what I like the most about Play!: it makes me productive. The development style is similar to Rails and I can simply get stuff done. I don't have to save my Java code, restart Tomcat, wait for Hibernate to fire up and then see if my code runs as expected. With Play! I make my modifications, hit refresh in the browser and presto! my new app is either up and running or there is a nice, pretty error message.
Running Play! is a breeze. The framework comes bundled with Jetty so I don't have to install and maintain Tomcat locally and when I'm ready to put the app into production, I just push it to Heroku and they take care of the rest.
If you want a great intro to Play! and why you might to use it, check out the first 10 minutes of this video from Dreamforce 11, "Introducing Play! Framework: Painless Java and Scala Web Applications". The reset of the preso gets into actually building an app so if you want to see some code, feel free to continue watching.