Many (most?) of the best programmers I know despise IDEs and rely solely on Emacs. I am not quite so diehard, and use both Eclipse and Emacs, but there are a couple of reasons I prefer using Emacs:
Consistent experience with ANY language - Support in Eclipse for some of the newer languages (Scala, Clojure, etc.) is inconsistent at best, and subpar at worst. The plugin authors are hard at work improving the experience, but I often run into little things that feel janky. Sure you can always switch to another IDE which has better support for that particular language, but now you are juggling IDEs. I've never felt this way while working in Emacs.
I dislike IDE "magic" - IDEs tend to do a lot of stuff automatically in the name of convenience. This is great...until something breaks. Then it can be a frustrating or even infuriating process getting things back on track.
I've been toying with the idea of switching to Redcar editor. It's extensible like Emacs, but instead of Emacs Lisp you use JRuby. It's lightweight, and definitely not a full blown IDE. If you are into TextMate, it's compatible with TextMate bundles, but it has the benefits of being free and open source. Definitely a tool worth checking out, especially for Ruby developers.
