On most cars, the cruise control only operates on the throttle, so if the car is going down a hill of any appreciable gradient it will speed up in the same way as a car without cruise control. If the hill isn't too steep, engine braking will keep the speed in check. This is how the cruise control on most of the cars I have owned or driven recently behaved (BMW, Lexus, Vauxhall, Audi, Range Rover).
The Mercedes E-Class however, has some kind of electronic control over the braking system, so the cruise control can actually apply the brakes gently in order to maintain a speed downhill, and will actually slow the car down quite quickly if you select a slower speed. It also has a limit function which prevents the car from exceeding the selected speed. This too applies the brakes if necessary. This is definitely the best cruise control I have used on any car, but it takes quite a bit of getting used to, even if like me, you are used to using cruise control.