I know the FCW doesn't apply the brakes, and only alerts you, but I had the ACC set so it alerted me as well as applying the brakes at partial force. Since I posted, I've still been using the ACC and it works just like it always did, but I've got my feet much closer to the pedals just in case. In normal around-town driving, I've had just the FCW go off when a vehicle cut across in front of me at a similar distance. I'm assuming the bag was somehow inflated just enough by the wind for the sensor to recognize an object, and it floated directly across the lane, so the motion must have alerted the sensor.
I don't think ACC is a foolproof system at all, just a convenient aid in certain driving situations. On my daily commute, about 20 miles each direction, 99% highway, I probably turn the system on and off about 5 times in one trip, because I'm used to the way it reacts and I've learned what situations confuse it, and anticipate them.
As far as the system braking when a car is exiting but not fully in the exit lane, mine does the same thing. It makes you look like an idiot to fellow motorists, but the car is just waiting until the lane is completely clear before accelerating. Not ideal, but imagine the trouble they'd get into with a car accelerating before the lane is clear!
When I first got the car, I would usually use distance 2 or 3 (tailgaters are one of my pet peeves), until I realized that any distance greater than 1 is an invitation for people to cut you off.
Finally, the Rover behind me was a nice Sport Autobiography, so I do believe it was equipped with ACC. Being a Rover, I'm sure there was a wiring issue and it wasn't working at all. Another reason I'm glad I drive the American Range Rover, even if it does occasionally think I'm about to have a major collision with a grocery bag