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!