I have one installed on my r56 Mini Cooper S and it works wonderfully. Even with the "stock" sport button there is a huge delay in engine response to throttle position; however, with the SprintBooster in its red setting all of the delay is gone and WOT is easily achieved at only partial throttle (<50% pedal travel) so the car feels much more responsive. In the Green setting the throttle isn't nearly as sensitive so it's easier to modulate the throttle but still a huge improvement over stock. The nice thing about the SprintBooster is you can turn it off (back to stock) which allows you to easily see the improvement in throttle response with each setting.
Also had one installed on my wife's old 330 and she loved it - she really noticed a huge improvement in throttle response, especially in reverse. We had it removed two weeks before we sold the car and she hated driving her car without it!
I've thought about getting one for my e92 M3 but with multiple throttle setting already I'm a little concerned the SprintBooster might make the throttle too sensitive. I will say you'll definitely notice a difference once it's installed - I bet with the Red setting you'll be able to achieve WOT at only ~30-40% of stock throttle travel!