I do know from my experience with supercharged vehicles that many of those have a nasty habit of not downshifting soon enough and laying into the boost when in cruise. I usually did not use cruise when in that type of terrain though and made sure it downshifted to minimize use of boost. Not doing that could cost me 2-3 mpg. Higher RPM but 1/3 of the boost will use far less fuel than lower RPM with a lot more boost. The difference will be far less with N/A motors, but it's not good for automatics to put a ton of load on the transmission when it's in an overdrive gear. After driving several other cars that did not have cruise, I can keep my foot out of the pedal pretty well and make my transitions to and from the throttle pretty smoothly, but cruise definitely has the advantage of keeping me from speeding (except coming down hills).

Yes, I was assuming brand new cars for those tests, as those types of magazines are typically geared towards new cars. I do still use the cruise in my truck, but mainly to avoid the aforementioned speeding tickets.