I was new to Play and just wrote a small-mid sized Project with the Java side of it.
It works and is powerful, but a lot of Features just seem to be very Scala centric and the Java support is sub-par.
The Java API Doc is virtually non existent, which takes away some comfort in the IDE (IntelliJ - I recommend it).
The integration in the Java world is not very "natural" (SBT..), It does not build as a war or maven module (but there seem to be plugins to do this). You kind of loose the benefits of the existing Java tooling.
The concepts which differentiate Play from the other Frameworks almost all seem to be scala native and Java second.
I stumbled onto some bugs and quirks and I always would end up in Scala code I had to read and understand to fix or work around it. (Actions, RequestBody stuff, Async, routes, template implicits and so on)
So the Java side of things just feels very second tier to me, although they are clearly making an effort for them to be equally good.
Now, after the Project, I would say it is very usable and would even use it again (in Java).