"Don't use cruise control it'll kill your mileage."
Since the dawn of feet-free driving this has been the case. hypermilers will tell you exactly how to change your driving style to get better mileage. I've always like the analogy: Drive your car like you would ride your bike. Or drive with soft feet (mighty ducks) like you have an egg between your right foot and the accelerator. That and many other tips can get you 10-15% better mileage than those silly EPA estimates.
Ford however is in the process of working on a better system. Full story here
"Dynamic cruise control "will automatically vary the vehicle speed based on the predicted grade. When a decrease in grade is predicted, the torque will be minimized ahead of time to conserve fuel, and this includes coasting and neutral when allowable," according to Ford research engineer Tony D'Amato."
So basically cruise control will now vary your speed and gear/torque to get you that 10-15% without even trying. It's about time.