I've recently started using the rails_rcov plugin to test how well I am testing the code I am writing. It's been a great help, but I was getting very strange results until I read this post about how to configure rcov properly to concentrate its testing. (Basically you have to force it to run your unit and functional tests in isolation which seems counterintuitive). It's easy enough to add parameters to your rakefile that will straighten it out.
Now I am going to try to get Heckle working for even better testing of my tests.