I get better MPG with the cruise off. The reason for me is that I can allow the car to slow down on uphills and re-accellerate on downhills without wasting gas trying to maintain speed on uphills. That's where I get the most gain. I have not noticed much difference between using cruise or not on the flats.

I have found that if I just keep my foot on the accelerator frozen in one spot then I get the best mpg - sort of a crude version of cruise. If I hold it still on uphills there's no extra gas to maintain speed so the car slows on the way up, then accelerates on the way down, then levels off on the flats. Works pretty well for me. Now that I started this technique I am doing about 3mpg better than before.