It might be worth trying the ScalaIDE Eclipse plugin - I think it works with Helios now. That said, I had so many issues using it half a year ago I went and learnt to use emacs with scala-mode, which, combined with sbt ~test, has worked out brilliantly.

The only thing I'd really appreciate an IDE for is mass refactoring (my project is at a point now where it's all a bit badly organised).

IntelliJ's IDEA IDE has excellent Scala integration in the Community Edition (i.e. the free version).

I've been working with Scala for about two years now and in my experience IDEA has always been ahead of Eclipse for Scala development. That being said I prefer IDEA for writing Java code too so this may just be personal preference.