actually the biggest benefit i saw when using cruise control is that since it only changes speed very slowly, if forces you to anticipate traffic a lot more. when you know you cant speed up or slow down very quickly, it makes you constantly look for traffic flow trends.
i live in an area where going anywhere on the freeway is a start, stop, slow down, sprint for openings experience. using cruise control in those situations makes you learn to anticipate better.
the hills and winds is another reason why i use CC all the time.
maybe its because i went from driving a ford F150 to the Prius, but i found that left to my own judgement, i tended to drive 10-20 mph over the speed limit. the Prius had such smooth acceleration especially on slight downward grades that i had never really noticed before. the car effortlessly gets to 85 mph without changing the pedal pressure if im not careful.
i'd rather stay in CC and see how many 99.9 mpg segments i can achieve at 65 mph!!
i went to Mt Saint Helens last week too except from Olympia, WA and i hit WA state route 505 at 53.5 mpg. got to the observation point and was at 49.1 (first time below 50 mpg since 1st tank of gas) but coming back i got to 53.2 so i got back nearly all of my mileage too. keep in mind that i was in a hurry to get back and had clear sailing nearly all the way down.