Just installed mine tonight. This may sound a little weird, but the car coasts a lot more when you're off the gas. Before when I got off the gas, the car would slow pretty substantially as if I were on the brakes. Now when I let off the gas, it seems to coast more. This should translate into better mileage. Pleasantly surprised.
That and the awesome "voooosh" sound as the M V8 does unspeakable things to the unsuspecting air