We use Play Java in production, in fact it plays a central role in our backend. Our entire persistence layer is written in Java.
Going forward our new code is all in Scala. Not because we ran into issues with Play + Java, we just get fed up with Java's verbosity when dealing with futures and actors.
A minor demerit was getting SBT and Play Java to work correctly in IDEA was enough of a pain to make me wonder how much overhead that was going to incur in the long run.
If someone who has used Play Java in production on a large project can weigh in on whether these points are true or not in real production use, I'd love to hear it.