I've been using TextMate for Rails development ever since I started programming on a Mac. I've got to say, though, that after trying out the new RubyMine IDE, I'm considering upgrading. Don't get me wrong, I LOVE TextMate. It's been nothing but good to me, and I still plan on using it for Scala and other editing tasks. That being said, my first IDE when I was just starting to program was Eclipse, and in that regard RubyMine feels like I'm coming home.
Cool things I've liked so far:
1) GUI test running reminiscent of JUnit. Yeah, I know, it's just eye candy, but what can I say? I like it, and my preference should count for something if I'm the one doing the development.
2) GUI Git integration. Point and click, baby!
3) intellisense/code completion/inspection/warnings. These can be annoying if things are too bad. Your code looks like it's been torn apart by a highschool honors english teacher, past a certain point. But overall I really prefer having the analysis and information built visually into my editor (another eclipse throwback).
Somethings that make me want to stick with TextMate:
1) Speed Compared to RubyMine, TextMate is blazing fast. It loads quickly, searches quickly, and I definately feel more nimble.
2) Simplicity. TextMate is an editor, and a good one. Your screen isn't cluttered with a hundred helpful windows.
I guess I've got some thinking to do before I lay down any money, but I'll post what I decide when I get around to it.