Fact = cruise control uses more fuel. It does whatever it takes to keep you at the set speed without any intelligence from the surrounding landscape i.e.: crest of hill.
Take an example - my trip home on the M1 going up the hill from Hawkesbury Bridge to the speed cameras. If I set my cruise at 100kmh, the the car will rev at around 3000rpm to maintain it and will often jump to 4k or 5k to maintain it.. as soon as you top the rise at the first speed camera you are still revving at 3000 and inching towards 105 until the "brain" realises it needs to relax..
I can maintain revs around 2000 - 2500rpm if I am in control of the car acceleration and as boatboy says - releasing your foot as you approach the crest allows momentum to carry you at a constant 100kmh.
I do this every day and have done for 20 years so it's not like I am making the stuff up..
