<http://www.radrails.org/> (Standalone download, or Eclipse plugins) At one time the standalone download was generally considered to be better than the Eclipse plugin, but I haven't checked back in quite a while.
Hence, I am not David, but I can tell you what he said regarding just such a question. At RubyConf last year, he basically said that with Ruby, a good text editor goes much further than it can, if you're programming Java. Also, the entire Rails core team uses TextMate. So, I'd give TextMate a little more time, before I switched to RadRails.
Ruby IDEs are getting better and better. I particularly like Textmate for Rails development, but it's only available for the Mac. The Eclipse plugin is pretty good too.
However, one feature I greatly miss is Refactoring support, and I mean something more than simple search & replace. It'll probably take some time to get a decent refactoring support for Ruby, given the dynamic nature of the language.
I prefer the look and feel of Textmate, its power, and minimalist appearance over RADRails. On a Mac machine I will use Textmate hands down but when it comes to windows or linux machine I will use RADRails no questions asked. RADRails is the perfect ide for those who are looking for an opensource multiplatform solution.