Depends how you use it. If you select 70mph on a motorway and choose instant fuel economy readout on your dashboard, you can see the fuel economy drop as the cruise control maintains your speed up inclines. Then, going down inclines, it brakes the car when it would be more fuel efficient to simply lift off and let it choose its own speed. If you were driving for economy you would lift and reduce speed going up inclines, then lift off entirely going down them. On the flat, it makes no difference and again, you can see for yourself on the meter.