I use Eclipse. For a while, during the Scala 2.7 to 2.8 transition, the Scala support was so bad I actually switched back to Emacs. Now it's fairly usable, though not as full featured as the Eclipse's Java support. I would also highly recommend you setup JRebel with Lift. It makes the development cycle go much faster.
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Which IDE do you use?
If you're an Emacs user, look into Ensime, it's fantastic. http://github.com/aemoncannon/ensime
I use Eclipse. For a while, during the Scala 2.7 to 2.8 transition, the Scala support was so bad I actually switched back to Emacs. Now it's fairly usable, though not as full featured as the Eclipse's Java support. I would also highly recommend you setup JRebel with Lift. It makes the development cycle go much faster.
The best overall Scala support in an IDE is currently in Intellij IDEA. The Scala plugin works in the free, community version as well.
I've enjoyed netbeans scala support.