I've been using Scala for about a year now. Started writing some test using ScalaTest, ScalaCheck, and Specs, and really liked it. Then I solved some hard problems using Scala, with a lot less code, and a lot fewer bugs, than it seemed like we would have ended up with using Java. I'd say about 99% of our codebase is still Java, but the Scala bits are quite noteworthy both for leading to something nicer to maintain over time (mostly by need no maintenance, or very very little), and for solving a hard problem without an explosion of Java boilerplate. A more functional approach ends up being a bit easier to test. The collections framework is >awesome<. For us it has been a slow, grassroots thing, and I don't think we are going to see a big bang, but those of us who have taken the plunge have a very hard time going back to Java.