Now I’ve got a few miles, the ACC is really good. Coming out of London on the elevated section of M4 which has a 40mph limit, with Driving Assistant Plus - the car drives itself. Even in this very stop/start traffic. At higher speeds there’s just the minor inconvenience of steering accurately, but the decision making and execution is just right.
The only time the system gets caught out is when I get cut up - sudden lane changes by a vehicle in front are caught but late and dramatically. I have gotten used to intervening when this happens, nowadays.