As an former Eclipse user I had to switch to IntelliJ for the last 4 months and use it on a daily basis. I was very surprised how well the Scala plugin works. Excellent code completion, code search and refactoring options. I tried also Scala IDE in the Coursera Course and think the overall development experience of IntelliJ (with Java and Scala) is much better.
you can edit the pom.xml file, and have your project adjusted immediately. intellij will download and add the new jar to the classpath... BAM
intellij is really a much better experience in developing than eclipse. Lots of fit and finish things, also intellij is very keyboard-centric, so whereas eclipse had lots of graphical widgety things, intellij has all your normal day to day stuff fully keyboard accessible. It's the closest to emacs in an IDE I've ever seen.
