I’ve used both PHPUnit & SimpleTest and I found SimpleTest to be easier to use.
As far as TDD goes, I haven’t had much luck with it in the purest sense. I think that’s mainly a time/discipline issue on my part though.
Adding tests after the fact has been somewhat useful but my favorite things to do is use write SimpleTest tests that test for specific bugs that I have to fix. That makes it very easy to verify that things are actually fixed and stay fixed.