I wrote about how I spent much of Sunday streamlining my writing setup (Latex, TexShop, my own shell and Ruby scripting). I continued this process by upgrading to IntelliJ version 6. I was using a trial update and found the new Ruby support and other small changes worth the update price from version 5. While TextMate seems best for working on small Ruby projects, the new IntelliJ Ruby and Ruby Rails plugin works very well for me on larger projects. For Mac users, TexMate is a "must have" tool. Even when I am developing Common Lisp applications (I use Franz Lisp + Emacs + Franz ELI), I keep the entire project folder open in TextMate - choosing to only keep Emacs buffers open for the files I am making serious changes to and using TextMate for browsing and short edits.
Before spending the money for an IntelliJ upgrade I revisited Eclipse for Java and Ruby development and the latest code drop for NetBeans 6 (testing) that has good Ruby and Rails support. In the end, I went with a supported commercial product. BTW, I find it interesting that the three most popular Java IDEs now support both Ruby and Ruby on Rails development - that tells you something about the momentum that Ruby has. I enjoy Ruby so much that I have all but stopped using Python.