Well, as I always tell people who have me install cruise for them, it all depends on how you drive in the first place:
If you're already a very careful and diligent user of the right pedal, then it's unlikely that cruise control will improve your fuel figures. However, if you use the accelerator more like a switch, and you tend to creep up above the legal limits on motorways, then cruise will certainly help to get more miles from a gallon.
As to how much of an effect climate has on consumption, it's anywhere between 4 and 10%, but put it this way, having a window open will hit you harder.
Cheers,
Mike