Set aside the IDE vs editor topic, one of the biggest complains about RubyMine is its performance. In a large project, without tweaking it a bit, RubyMine could be so sluggish that it becomes simply unusable. That's probably the #1 reason many RubyMine users ditch it and go back to simpler lightweight editor such as vim or TextMate. Well, there are actually a couple of things that can significantly impact the performance of RubyMine. With a little care, RubyMine can perform with an OK smoothness.
1) By default, RubyMine's jvm max heap size bound is set as 512MB, it's probably be enough if you are running it on a 32bit JDK, but on a 64bit JDK, it might be necessary to bump it up to 1024MB, since the 64bit uses twice as much memory. This will reduce the number of GC collection. Here is some guide on how.
2) More importantly, RubyMine index everything in your project to support text search/refactoring etc. So if you have some huge files that also get changed very often, such as log, spec reports, etc, it could be very expensive to have RubyMine indexing them all the time. Exclude them from RubyMine project structure is probably a good idea.
That's it. I think with a little love, RubyMine can still be a valid ruby development tool, especially for people who are used to JetBrian's IDEs.