A lot.
You'll find yourself changing gears much quicker, and a lot more responsively and when you plant your foot the changes will be very sharp and you'll get a little chirp from 1 - 2, sometimes 2 - 3, but in the wet it's anything but a chirp.
When your foots not planted its normal gear changes, but its a very noticable upgrade when you want it to be. Personally I wouldn't recommend installing it yourself. But i'd definitiy recommend getting one.