The adaptive cruise control in my Ford turns itself off if the speed is low enough-- which on Seattle-area freeways happens pretty often. So I stopped trying to use it.
The Subaru one stays on until you come to a complete stop. Even then, it's still "on" in a way, since it'll hold the brakes, but you have to flick up on the cruise control switch to get the car to start moving again. It'll also notify you if the car in front has started moving again and you haven't done anything (with or without ACC on). It's on my girlfriend's car, so I don't use it every day, but it has been nice the times I've driven it in traffic.