I don't have one myself, but I have read a lot of opinions on them. You can find tons of info on this product if you do a search here or on
www.probetalk.com
Most people seem to be very happy with the results overall, not a big power boost, but a little bit and improved throttle response. The main problem, and the reason I haven't gotten one is that you have to grind a bunch of material off the mounting point on your engine for the pulley to fit. It's not that big of a deal, but I don't really feel like taking a dremel to my car. Also, if you have a big stereo system, it can cause your lights to dim and what not, since it is driving the accessories slower.