Yeah, like much Rails-related stuff the documentation is of the sparse and poor variety. However I did find that the rdoc with the gem for the Test::Rails portion of the ZenTest suite just sufficient enough to get started using it.
I don't see a lot of use for the "ZenTest" executable, but then again I tend not to produce the kinds of gaps it detects inthe first place. I do find autotest and Test::Rails useful though, when autotest behaves that is.... (It seems to like to get the testing database into a funny state. as well as forgetting that modifiying the test file or the production file should generate a rerun.)
My RoR journey
-- thoughts on learning RoR and lessons learned in applying TDD and agile practices.