I have been using the pre-release versions of RubyMine occasionally, just to try them out. The new new 1.0 beta is a large improvement, mostly because of much faster responses while editing, running tests, etc.
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:
Now 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.