I agree with the above. As a Java and Scala programmer I've tried both and Play! 2 Scala is a joy to use if you're fond of idiomatic Scala and functional programmming.
I use Eclipse with the Scala IDE plugin and although I write all of my code in Scala, I do have a couple of dependencies to Java libraries (the interop works perfectly, but of course you should be careful when mixing immutable code with potential sources of mutability). The Intellij plugin is probably still the more stable of the two, but as mentioned by yareally the Scala IDE is the officially maintained one so you can expect new features to come to Eclipse before Intellij.
I use Eclipse with the Scala IDE plugin and although I write all of my code in Scala, I do have a couple of dependencies to Java libraries (the interop works perfectly, but of course you should be careful when mixing immutable code with potential sources of mutability). The Intellij plugin is probably still the more stable of the two, but as mentioned by yareally the Scala IDE is the officially maintained one so you can expect new features to come to Eclipse before Intellij.