My wife gets better mpg w/ CC on. Because otherwise she's speeding up, then slowing, then speeding up, etc. The large swings kill FE, especially when the high end is >70mph. So for her, keeping CC on set to 65 improves on what she was getting.

I drive without CC, but keep speed/power very constant, looking to drive with the load, draft, neutral coast, etc. So I get better mileage without it.

If you tend to drive with a lot of speed swings at the high end, CC will be an improvement. But if you drive 'in synch' with what your vehicle wants to do (most of the techniques here), CC will not be as good as that.