I just switched in the last couple of days from Eclipse to jEdit for Rails development, following the tips here. I knew jEdit was a nice editor on Linux (about on par with TextWrangler on Mac OS), but never seriously considered putting it on my Mac -- or considering it an IDE -- till I read that article.
As much as I like Eclipse for other languages, I am coming to the conclusion that it's overkill for Ruby, and that the Aptana team has not really followed through on RadRails' early promise. jEdit has Ruby code completion and Haml syntax coloring modules that work now, not whenever the bugs get around to getting fixed. jEdit's UI isn't quite as nice as Eclipse's, but it's a lot less bloated and more responsive.
(And before you ask, I really don't understand all the fuss among Rails developers about TextMate at €39, when TextWrangler is free and seems to be no worse, and jEdit and -- yes -- Eclipse are free and apparently better. Would someone care to enlighten me on why it's worth the €39?)