It's nice on the M3 because you can turn it on when you want it. I've used it when stuck in bumper to bumper traffic.
However I had an X3 loaner and I HATED it for two reasons:
a) Pulling up to a roundabout. As I slow down to see what traffic is coming to my left I am turning the wheel getting ready to turn into the roundabout. F*ing car turns off and of course you can't turn the wheel. You can bet I was cursing at the car at that point.
b) Attracts way too much attention when driving around the city. At every intersection people are walking by looking at you like 'WTF' as it sounds like you are turning your car on and off.