Cruise should give better MPG because it stops the driver changing speed a lot. If I drive without cruise I tend to speed up over time to 'keep up with traffic' then realise I'm going quite rapidly, then slow down again, and repeat. With cruise, I set a speed I'm happy with and stay there, just changing lanes as needed, and adding a wee squirt to move past traffic that is doing a near identical speed.
I use cruise on motorways, and in average speed roadworks a lot. No point on back roads or on congested A or M roads.
I did the 128 miles to NAD this weekend with cruise pretty much all the way. Set to a comfortable speed 'keeping up' with traffic, and returned 35mpg over the trip from a 3.2 petrol. Without cruise it'd be in the high 20s.