Sorry to resurrect such an old thread, but I just picked up a 2012 Genesis yesterday, and several times already, I thought I was going to be rear-ended due the ACC slamming on the brakes in traffic on the highway. I understand it was trying to keep a pre-specified distance, but in my opinion, it did not need to slam on the brakes to that extent. It's like it was hedging it's bets, on account of the possibility the car in front might suddenly slow dramatically, and braked more than it really needed to.
Just wondering how long it took you to adapt

to the ACC. So far, I don't really like it, because it is always slowing me down before, in my opinion, I really need to and to a greater extent than I need to. Then after I change lanes, the car has to speed back up, thus reducing my gas mileage. Bear in mind that I have not had an accident or traffic ticket in over 20 years, so my driving record is pretty good all by my lonesome.