I've been using JUnit Max for the last few days. It's an Eclipse extension that automatically runs your tests every time you save a change.
Normally I'd stick to open source options (which this is not), but I figure Kent Beck deserves some support.
It's taking me a little getting used to - makes me nervous not running the tests manually. It's designed to be unobtrusive, but part of me wants more blatant feedback that my tests passed.
One of my motivations for using it is to make sure I'm running all the tests, not just the one I'm working on. I've been caught a few times where I didn't discover I broke something elsewhere till some time afterwards.
Even with a fast machine, it still takes a while for the tests to run (although I'm the only one to blame for the speed of the tests), but at least they run in the background and I can continue working.
The web site is pretty minimal and there is no documentation as far as I can tell, but then again, it's pretty self explanatory.
One of the things that prompted me to try it was listening to a podcast with Kent Beck on Software Engineering Radio (recommended).
JUnit Max has had a bit of a rocky history, pulled in 2009, relaunched in 2010. I think it's a worthwhile product and I hope it's successful.