cruise control actually controls your gas pedal/throttle plate. SOme cars have it attatched to the brakes, but not many. If you don't like your speed going downhill, either hit your brakes (which SHOULD disengage CC) or turn it off and then hit the brakes.
The engine randomly revs ont he freeway becasue the comptuer determined that it needed to increase power to maintian speed.
as for gas mileage, cruise control's effect on it is a subject of heated debate. I suggest just using it on the highway where it'll be of the best use.