It makes a big difference on my Z4MC, mostly to the throttle response and in a way that can't be achieved just by putting your foot down further.
It's a good button Brent.
I find it makes for greater precision in throttle, each millimetre you push the the accelerator you feel the impact, makes my car respond like a Scalextric. I find it is too much to use it on the road unless you want your passenger to be sick. I've used it on windy roads before and it's not comfortable for long periods of time, made me feel a little queasy myself.
Great on a track.