My use of ACC is similar to that described by jadnashuanh. When I see vehicles stopped ahead, I will usually momentarily disengage ACC as jadnashuanh described to control deceleration myself because ACC's deceleration can be stronger than I prefer, even to the point of using friction brakes because regen braking isn't sufficient due to deceleration having been started too late by ACC. I think some of this is due to the limited forward range of ACC's rather low resolution VGA camera which results in ACC not recognizing stopped vehicles ahead as soon as I do.
I have never experienced the warm-up period described by bwilson4web. Our ACC works correctly immediately upon being turned on. As bwilson4web described, our ACC camera can be blinded by direct sunlight entering the camera just like our eyes.
On a couple of occasions, ACC did not recognize slower white box trucks ahead. Had I not intervened, ACC would have happily driven our i3 into the rear of these trucks, so one must stay alert. I now pay attention to the display that shows a red icon of the rear of a car when ACC has detected a vehicle ahead. If this red car icon doesn't appear when I think it should, I prepare to intervene.
Other situations in which ACC isn't perfect is when a vehicle changes out of my lane. ACC will frequently decelerate unnecessarily even though the vehicle has completed the lane change. I usually rest my foot near the accelerator pedal so that I can press the pedal when ACC decelerates unnecessarily, or when ACC is sun-blinded, fooled by a dark underpass surrounded by bright surfaces, or when driving on a windy road with vehicles parked along the side of the road.
ACC should be viewed an a driving aid only. The driver must always be ready to intervene. Even with these imperfections, driving with ACC is much more relaxing, and I would never buy a car without it.