I got one on my old Rover 214 and can honestly say it really worked!
I out-dragged a Skyline Lemans - supposed to be nearly 900bhp. Walked him like he stood still. I then entered F1 and won the world championship, smashing every lap record for every circuit ever made.
I did it all one only half a tank of petrol too, because I get such good economy I never need to fill up. And all for only £4.99 (plus £2.95 p+p) from ebay.
So don't knock it until you've tried it...