Thought some people may be interested in the following info / threads I have just come across myself. They are regarding what seemingly is the fast upcoming support for Ruby/Rails within the NetBean's IDE. I'm personally real interested in the auto-completion. It should be noted the Ruby/Rails support is still not finalised into a stable public release, so I've downloaded the latest daily build.
Extract from last link above: Netbeans owns its competitors in Edition and Navigation, which is after all most of the time you'll spend in (apart of debugging). The syntax highlighting is very effective and clear. The autocompletion is advanced next to Idea and Eclipse at the cost of Indexing the Ruby and Gems SDK at first run (few minutes). The filtered autocompletion is in progress and depends on the context. Once you use autocompletion a list with all possible functions will appear, and while you choose any of them, you'll get the associated RDoc, saving you precious time. It seems as if the browsing between class or through hyperlinks were better than its competitors. A debugger should be available with Milestone 8.
I'm sure the Sun team would appreciate any feedback on the Ruby support features as they aim to complete it and roll it into a stable release.