I have been doing a lot of Ruby development lately (a customer job and my book project) but I have mostly been using TextMate. In the past, I used NetBeans+Ruby a lot, but it was just not responsive enough (but a great IDE, none the less). I am too busy right now to change anything in my workflow, but when I get a chance I will switch over to using RubyMine.
Code completion works great for built in classes. It also seems to catch just about everything that it can from local code context. For example:
class StringNow code completion on a string or variable set to a string picks up the Stemmable mix-ins. Yeah!
I also like that RubyMine come pre-configured with cvs, svn, and git.