The OP talks about Dynamic Cruise control, which is standard across most of the newer BMWs. It will apply the brakes if the vehicle exceeds the set speed, or if you want to decelerate using the stalk. It is NOT ACC because that option allows you to set a distance from the vehicle in front of you, and will accel/brake in order to maintain speed and distance. This isn't avail on the X5 and I have no idea why.

I hate the new cruise control stalk though. They should put it back on the steering wheel where it belongs.