In my professional experience with large, very long-running Ruby projects, RubyMine will at least once per session (and usually much more than once) give you search results that are no better (and sometimes worse) than if you'd just run `grep` on the codebase. (e.g. "I'm pretty sure ONE of these five hundred 'run' functions is the one you want! Oh, by the way, I also am showing you results from every .gem file I know about!")
It also has a bias for presenting results quickly and allowing you to make queries quickly... so quickly that it often returns partial results to queries with absolutely no outward indication that the results are incomplete.
Given what it seems that the other Ruby "Intellisense" plugins are having to do to well-understand real-world Ruby projects (especially those who use Rails and friends), I'm sure that a godawful lot of manual work and special-case knowledge has gone into RubyMine's Intellisense. But, it's still not nearly enough to be reliable on large projects.