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Just finished a quick project: play! was fast, easy, and fun to use.

How does it compare to Spring Roo? And a long shot - to Ruby on Rails?

Well, Roo is nice but it's not java anymore. It's AspectJ :( Ruby on rails is nice too IMHO much better than Roo but it's not java too.
The best thing on Play is that it tries to be simple (very similary like Rails does). A not just to be simple it forces you to make things simple !!.
Play is very great framework.

I think it's more comparable to Grails

Voted down in favor of Pojo

I thought you were all for native mode :-)

yes the native mode takes precedence over jvm, just like Pojo takes precedence over XML configurations

Well, you're out of luck here, then, because there is no xml use in Play! Just the occasional subclass and annotation, and a few unconventional but very handy idioms. Play! aims at simplicity and development speed; I think it has an upper bound of complexity, in the sense that there are some application so complex that Play! is not the best suited tool. But there is plenty of room below that.

You may be wrong, but you're for sure a bit confused: GWT is a client-side technology based on a java-to-javascript compiler, Play! is a server-side framework based on a few cool classloading tricks to implement in java the same fast-turnaround feeling of Rails and other modern frameworks. They're apples and oranges.