In a word. Excellent.
The only slight complaint I'd have is it's a bit sluggish at low revs coz I have the induction kit fitted. But that is inevitable really. Once it reaches 3750rpm though it goes like the proverbial rocket ship.
I just love the noise it makes. It turns heads and people just don't understand why it's so loud when it looks standard.
I even had some bloke come up and say "I like the new Cooper S" and then was dumbfounded when he couldn't find the bonnet scoop. Ha ha.
A little bit of explaination put him right though!