I have ACC and in another thread a few days ago I wrote that it is a gamechanger for me. I didn't realize how cool it was until I had a car with it. I use it primarily on my commute, and in southern Cal you can get into some stop and go traffic. It's not perfect, you do need to remain vigilant at all times as, in my experience, it isn't 100% and isn't always smooth in its operation. As others have pointed out, it stops more abruptly than you would when traffic is at a complete stop ahead. In terms of reliability in basic performance, it's very close to 100% and isn't like lane keep assist, which was so wonky I just keep it off all the time.
I've used it in the rain with no issues, I primarily use it on freeways but will also use it as a speed control (yes, you could do this with regular cruise too, but I never did with previous cars) as it's also good down to a low MPH following other cars. As others pointed out, it take you to a complete stop and in that case you tap resume or tap the go pedal. If you are still rolling, no matter how slow and traffic picks up, it picks up too. I do find occasionally that acceleration might be a bit faster than I would do to resume a speed once a slow car clears out in front of you and as mentioned, it brakes too hard in certain situations. As long as you are aware of your surroundings, remember you are still the driver, how close is the vehicle behind you following, etc. it works great and isn't dangerous IMHO.
I personally wouldn't buy a car again without it.