Netbeans IDE 6.0 Beta 1: (J)Ruby and Rails support is looking good; mixed experience with IntelliJ 7.0 + Ruby plugin
/Applications/NetBeans/NetBeans 6.0 Beta 1.app/Contents/MacOS/netbeansto explicitly set jdkhome to my 1.5.0 directory. Hopefully most people will not have that problem.
I found it most convenient to have local JRuby and Ruby installations in my home directory and use those so the NetBeans GEM tools had write access.
I still find using TextMate to be more convenient for development, but code completion and browsing features may win me over in the long run.
I have a 30 day IntelliJ 7 trial and installed the first Ruby plugin for version 7. I ran into errors running the generate script and could not create a Rails project from an existing Rails application directory. I would bet that these problems will be fixed quickly, so I will give version 7 another try before my trial license expires. (I am trying to decide whether to upgrade to version 7 and Ruby support will affect my decision).