Gotta say, the jury's still out, but overall I really like it. Lots less navigating around tests and hitting shift-ctrl-F9 (or whatever it is). I got inspired after listening to a year-old RailsConf talk by Kent Beck about the origin of TDD (after discovering we went to school together at UO).
There are other tutorials that walk through the installation, so I won't repeat them here. But the additional information you need is distributed in 3 or 4 posts across the net, so here they are collected:
The first trick is to get a couple additional gems (besides ZenTest, which is what you need for autotest).
You will also want:
sudo gem install autotest-rails redgreen autotest-fsevent autotest-growl
Autotest-rails teaches autotest how where tests are in Rails projects (as if there are any other kind). Without you won't see any tests running. RedGreen adds nice coloring. FSEvent makes it faster on MacOS X, and autotest-growl, well, growls.
Second, I didn't find a complete listing of what your ~/.autotest should look like on OS X. Mine looks like:
That's it! Definitely give it a shot. It's most convenient if you have another screen for the autotest (IDE integration would be nice). And an extra CPU wouldn't hurt, because it keeps my MBP CPU pegged much of the time.