I just downloaded and installed the Cruisecontrol.rb 1.0 release that was just put out by some of my old co-workers at Thoughtworks earlier today. It literally took about 2 minutes to get builds up and running with our project.

As far as I can tell, it seems to be exactly what you would want out of a CI tool, it's easy to install and it gets out of your way until a build fails. Good stuff.