I'm using IDEA solely for Ruby and liking it, except that it's sluggish compared to the competition (Eclipse variants, TextMate, and Emacs). I have an oldish MacBook: 1.83 GHz Core Duo, 2G memory. Some of the slowness seems to be disk thrashing, suggesting that 2G is not enough, but some of it seems to happen even when the Activity Monitor doesn't show disk activity (such as editing method names when autoscroll to/from source is turned on, or scrolling up and down, or sometimes simple typing). Perhaps that means that a mere 1.83 previous-generation chip doesn't cut it or that a graphics accelerator is needed.
Perhaps it's Apple's rather weak Java implementation.
What do other people see? (Keep in mind that the Mac is very popular among Rails programmers, even though TextMate is their trendy editor of choice.)