had a play last night, I didn't notice that much difference while driving, mind you I didn't push it very hard
I did find that I could get a much cleaner start from a standstill, when I normally attempt a rapid start I can a bit of jerking and 'bouncing' type wheelspin, but with ESP off it just lets the wheels spin, but its much smoother and I found it easier to control by practising feathering the throttle and getting the right balance. I was suprised when you consider I've got around 300bhp through the front wheels with the ESP off!
I think I'll definately be keeping it on for normal driving conditions, and definately in the wet, but I may be tempted to reach over and turn it off when I find myself sitting at the lights on a nice day!