Using NetBeans 6.1 here. Before that I tried GEdit with lots of plugins and Aptana on top of Eclipse. Think I'll staty with NB.
The whole idea of a IDE for a scripting language is wrong in my opinion. Most good developers will prefer to use a good text editor rather than something that's marketed as an IDE I've found. Being a Linux user, I tend to use gEdit and terminal sessions. If you use OS X, Textmate is pretty good as it's got a lot of decent features without the bloat / rubbish most IDE's tend to have. IDE's imo are only needed / useful when using a statically typed / compiled language where integration with a good debugger, type safety, etc are important.
To call Ruby, as used for Rails, a "scripting language" is what's wrong here. In a Rails context, Ruby serves as a "full" programming language, and as such, it deserves development tools to match -- including an IDE, at least for some projects. This is particularly true since Rails, as wonderful as it is, has an annoying tendency to spread closely related code over up to 5 or 6 files in different directories, so some sort of intelligent editor is a big help to consolidate things.
I've done other Ruby work (for example, Rubinius hacking) in which an IDE wasn't necessary. Rails development just isn't that kind of programming, I think. It may not cry out for an IDE quite as much as certain types of compiled programming would, but to say that it doesn't need an IDE because Ruby is a "scripting language" (whatever that is) is just silly IMHO.
I've tried Emacs, Vim, Aptana, Netbeans, Jedit, and Textmate. Textmate is the clear winner for me, with nothing else coming close. If I were a vim wizard, it could be close, but I haven't set aside the time to become a vim expert yet.
I am using Netbeans 6 as I have windows machine. It works well for me. I even used it while working with WATIR. But if you have mac then there is no better option than TextMate.
I've used Eclipse (Aptana RadRails), Netbeans 6.0, and ActiveState Komodo, and I've taken a liking to the last two. For some quick and dirty editing though, I still use the scintilla editor that comes with the Ruby installer. SciTE - along with a keystroke launcher like Launchy - provides a decent working environment for working with Rails.
Same ol' question... same answer...
1 up for textmate. Only for those who gets satisfied with an intelligent text editor and do not require an IDE.
Netbeans... get as new version as you can, even beta one. 6.5 will be great release, currently milestone 1 released but I use it and it doesn't crash.
I am currently "trying out":http://erolfornoles.blogspot.com/2008/08/editra-free-lightweight-cross-platform.html the "Editra":http://editra.org editor. I've been growing impatient with the loading time of the IDE I'm using recently.