On flat roads, the cruise control is amazing at maintaining a consistent speed, and therefore you get a good MPG. On the hills, it is a different matter entirely.
When drving a long distance, I do still use the cruise control, but when the hill is steep enough that the car is going to downshift, I put my foot on the accelerator where the cruise is at, cancel the cruise control, then give as much throttle as I can without a downshift. Some hills are steep enough where I let the torque convertor unlock but I still try to keep it in fourth.
Interestingly, when driving my minivan (weighs in @ 4K lbs empty) I found on the steeper hills, turning off the overdrive completely and holding a steady speed is more fuel efficient than allowing the car to slow down in fourth, then having to accelerate to maintain above 40. Below that, I find it impossible to get good gas mileage with an auto.
BTW a steep hill is one with more than a 10% grade
