Apologies to the RDT folks. The slight by omission wasn't intentional, just some foggy brain action on my part.
I actually used RDT a around a year ago when Charles and I were pair-programming on (what was then) the Ruby project that became Freestyle. Back then, if I recall, it was primarily syntax highlighting, but it also had unit test running, which impressed me quite a bit at the time for how new the project was. And yes, I'll do my penance and check out RDT more fully real quick-like, as we say down here in Texas.
Of course, in thinking about the topic, it seems like the de facto IDE for Ruby -- at least among the vocal crowd -- is TextMate. I like TextMate myself and after several rounds of playing with it have switched to using it for all my plain-text needs instead of my old stand-by, emacs.
The problem there is that TextMate is OS X only. So, while I'm lucky enough to work on OS X and, among many other apps, use TextMate, I'm eager to see the further evolution and growth of a cross-platform Ruby IDE for those who are on other platforms.
And, to use another Texanism, maybe the RDT crew will fix that all up ;>
Disclaimer: Eclipse, which RDT runs in, is a client. As is Sun.