I've tried Play and I'm impressed: it does a great job of delivering a useful development model that is far simpler than most frameworks'. More than anything else, the runtime's ability in 'development mode' to parse .java files directly is worth a lot: just reloading the web page in the browser without running a build script or waiting for a redeployment is worth a lot of development speed. The error messages shown in the browser are really good too.
Another thing that impressed me was the overall aesthetic: it is perhaps a small thing that the tutorial application actually looks good (both the code and the web page design), but this extends to the whole framework, the API as well as the documentation.
