Somebody on the Monte Carlo list just installed a Pete Jackson "quiet" gear drive in their car, and loves it. They did it solely for the sound though, which isn't annoying apparently..I can dig up the e-mail if you are interested...why are you interested in one is kind of important though, I can't imagine they are really necessary in most any application except when you want it to sound cool.
I guess the best part of them is that they will outlast the motor, so you can really get lots of use out of them.