I know a guy who has one fitted and was considering one myself a couple of years ago, but the price of them for what they are is very, very high. All they do is kid your throttle pedal sensor into thinking your foot is nearer the carpet than it is.
I read a review of one in Audi Driver magazine a while back and they said it was actually better suited to a remapped car than a standard one due to the power delivery characteristics.
They don't give any more power or torque what so ever, all that happens is the throttle will open wider for less movement of the accelerator pedal. Great fun in some situations, bit of a headache in heavy traffic and loads of wheel spin in wet weather. Maybe worth it for £25 but not for what they charge for them.