I do many journeys on the M4 driving 70-75 too and I always use cruise when possible. I have the IS250 and it usually returns 35 to 38mpg.
Something I'm unsure of though. In the steep areas (ie Sarn Park, Bath), it will drop a gear and often two down to fourth in order to maintain speed. I've not tried it, but I'm sure I could coax the car along without dropping lower than fifth gear. I'm not sure if the cruise control is basing its gear selection on optimum economy or not ?
Overall, I'm certain that the computer can operate the controls far more smoothly than my right foot, so I'll continue to use cruise. Report back with your experience please Nick.