I use it on A-roads that are known to be reasonably straight. On twisty country roads I want better control of the car. It works very well.
If I had to find a fault, it is if somebody turns off that the car is tracking, the car slows down OK, but when the car is no longer in front of you, it can be a few seconds before it starts to accelerates back up to speed. Not a problem for me, but the impenitent driver behind you thinks your messing about...