it's dependent on the drive..
I would say from experience that over a long distance motorway journey, if you can control yourself and your speed manually, you will get better mpg.

The problem comes with the constant change of speed and accelerations and decelerations of using your right foot to do the work.
The cruise control will help that side of things, but will not be the most efficient means of control.

Ideally, when driving long distance, there is a fine balance between too much and too little throttle to maintain speed.
You dont want to be accelerating up hill, and you dont want to be losing too much momentum.

I think that if you can control yourself and be very self aware when driving, you will most definitely get better MPG manually.

you CAN do better yourself.

I find it does better though, as it keeps my speed down, which has a tendancy to creep up and up when driving manually.

I only ever use it on motorways myself

yes and no, hills for example, with cruise mine is usually set at 80ish, if driving without cruise on your foot stays in same place, and you may loose 5mph going up a hill. Cruise will open throttle more so you maintain 80 up the hill.

But over the course of a long run I think its better as you don't accidentally drift up to a higher speed