More in my experience, especially on our roads in the UK.
I bought my Cruise Control thinking that it would save me fuel in the long run, but after experiencing it for a while I realised that a human in control of the throttle is more efficient than automation.
With it activated you are constantly trying to adapt your speed by:
1) Turning it on and off
2) Using the stalk thingy to attempt to control your speed up and down in a very unresponsive way
3) Breaking like crazy when you get it wrong!
Other than that when everything else stacks up for you, it lets you take the pressure off your legs and feet on the occasional long drive ...
But never think about it as a fuel saving device in this country ...
If only we had German roads ... mmmmm!