I just bought the Sprintbooster. It gives you a better throttle response, more like the old gas pedal attached to a cable. I read tons of reviews and everyone who's tried it is a believer. I'll let you know how it is when I get it on Monday. The lag from the 550i is so bad, I took my car in for warranty thinking it was a fuel delivery problem. They said they just reset my software. Still no good so I finally ordered the sprint booster.
Www.sprintbooster.us is 299 and free overnight shipping.