http://www.scala-ide.org has come a long way and can be usable and useful. I recommend a clean install of eclipse 3.5.2 with the latest recommended build of scala-ide. Try things out without installing additional plugins except your favorite version control support. I like the refactoring and code reformatting support!

I've recently started using the IntelliJ EAP X builds for a personal project in Scala and find it usable and useful too.

The maven-scala-plugin is solid and will let you switch between IntelliJ and Eclipse relatively easily. I don't use the maven eclipse plugin but instead "mvn eclipse:eclipse" from the command line whenever the pom changes. Using a pom-based build is useful in a production environment especially if you are using/providing components from/for others. IntelliJ has maven support built-in and seems to work well.

I've only pushed the IDEs to ~40 source files. Things might start breaking as the projects get bigger.

Things are definitely better than they were a year ago and thanks to both the scala-ide and IntelliJ people!

Haven't used IDEA/scala plugin in a few weeks, but i did have to mess with heap and other config options per:

http://news.ycombinator.com/item?id=1438408

http://www.assembla.com/wiki/show/snapimpact/IDEA_9CE_Tip

http://leshazlewood.com/2007/08/01/intellij-idea-on-mac-os-x...

I'm used to doing everything in emacs anyway, so the IDE issue wasn't much of a hindrance. I didn't find the learning curve all that steep either, although I do have some prior experience with other functional languages. Overall I found Scala an extremely nice language to work with.

I've put it on the back burner for now though because the job market is still so tiny. If I see more people looking for Scala skills I'll make it a priority again.