Ford commented on this at the "experience" event in Dearborn. Basically the cruise control is probably better than most drivers at milking the best fuel economy on flat land.

However... the cruise control can't "anticipate" what's ahead and driver's can. On hilly terrain, drivers can take advantage of gravity on the downhill slopes to accelerate (so it doesn't really take much fuel to accelerate), then ease off the pedal on the upslope and let the momentum carry you back up the next hill. Cruise control wont do that. On hilly terrain a driver who knows how to milk it can do much better than cruise control. On flat terrain the cruise control will beat most drivers for milking fuel economy.