This is the same question for older version of Scala, but they say that Eclipse plugin has been improved vastly. Is it the best IDE now? How do different Scala IDE compare t
I'd recommend IDEA's plugin for now.
The Scala plugin for NetBeans is quite nice too. It doesn't yet support NetBeans 6.9, the newest release, though, and you still need to download it manually instead of installing it directly from the plugin manager inside NetBeans.
However, it integrates better with Maven projects than IDEA's plugin does (this is true for NB and IDEA in general, in my opinion).