I find ACC very useful in certain situations.
For example, a local village to me is Henley-in-Arden, which is now subject to a 20 Mph limit throughout.
I guess like most drivers, I find it very easy to exceed that limit, so a quick flick on the button prevents me 'speeding' , probably much to the annoyance of some following drivers! (yes, it operates at 20 Mph, no problem)