Yes, Steve D'Gerolamo, the owner of ultimate garage, is very knowledgeable about BMW. His prices are fair, but definitely not the cheapest (go to BMA for *lowest prices). I like his approach of giving both sides.

That said, I wouldn't buy a cat from him or any other mail order place. I would find a local dealer/shop that sold DEC cats and get it from them. That way if there is warranty issues, its easier to take care of.