I have one, it is a Bel 9xx and it works great...I work at circuit city and it is one of the better ones we sell. I believe it was rated the second or third best behind some massively expensive escort type!
As far as performance, saved my ass dozens of times...Zero tickets with it on my car for almost three years...I don't speed though
One damn ticket while it was sitting in the back seat, that'll teach me!
Chris