Cruise Control does a much better job at holding a constant speed that a person can; the steady speed is needed to give you the best mileage. I drive more with the cruise on than I do off; even in rental cars, when I'm going through the initial "figure out where everything is before you drive off" check list I go through, one of the things I always check is "where are the cruise control "switches"?Of course if it's optional, then don't pay for it. But if it is standard equipment then I don't see what's wrong with having it on? You pay for it either way, might as well get resale value out of it.
For something like cruise control, I think your milage will be more impacted by having a large lunch then by having the cruise control mechanism installed
Someone else in here said it best; "driving with cruise lets me watch the road and not the speedometer".