That's a good way of explaining it.
But going up/down hills is only a small part of the story.
A lot of it comes down to how well you concentrate on behaviour of traffic far, far ahead, how you analyse what's going on to anticipate what's going to happen, and how well you use the accelerator to adjust speed in advance before everyone else does ("acceleration sense" as Roadcraft and advanced driving people call it).
Be prepared for me to bore you to tears, the following is going to be mighty dull reading
Keep in mind the following principles:
1) Whenever you reduce speed by lifting off the accelerator, the engine uses no fuel.
2) Every time you brake, you've wasted fuel - because it cost fuel to gain that speed in the first place, and unless you slow down by lifting off (and cutting fuel) you've effectively wasted it.
Consider the typical driver, using cruise control:
Scenario 1....
You're on a motorway, with a light amount of traffic, you're following a long but spaced out procession of cars in lane 2 at a steady pace. You are running on cruise control. It's a typical day; you're maybe thinking about work, listening to something on the radio, perhaps talking to a passenger. You notice brakelights of the car in front, or maybe you see the high level brakelights of cars 2 or 3 ahead. You need to slow down slightly, so you brake, killing the cruise control. Cars behind you brake too.
Scenario 2....
You're driving in a single-carriageway national speed limit road with cruise control set to 60mph. There is a 40mph limit ahead. The cruise control pushes the car along at 60mph. When the driver gets near to the 40mph signs, he brakes to bring speed down, killing the cruise control.
Consider the skillful driver, not using cruise control:
Scenario 1...
Same motorway, same traffic, same scenario. However. Your mind is 100% on your driving. You have positioned your car so that with a combination of good distance from the car in front, and a slightly right-of-centre position in your lane, you can see what's going on a long way ahead. You've been watching the typical driving styles and reactions of the drivers up ahead of you. You've already spotted that a couple of hundred yards ahead in lane 1, a truck is closing the gap on another truck in front of it and is likely to pull out to overtake, but it hasn't started indicating yet. The cars in lane 2 behind where the truck will pull out are approaching it at a higher speed, which means they will have to start braking soon. This will cause all the other cars behind to slow down. Because you know this is going to happen, you very gently start to lift off the accelerator, and you gently lose speed, opening up a slightly larger gap ahead of you, while the typical drivers ahead of you maintain their speed. As expected, the cars ahead do start to brake because they're all typical drivers, event-driven by the sight of brakelights. When the brakelights of the car in front of you come on, then - no drama - you've already dropped back and adjusted your speed and you don't need to do anything; when the car in front has braked down to the desired speed, you're back at a normal distance behind him. But at no point have you touched the brake pedal, and you feel smug
Also, you've smoothly controlled the speed of the cars behind you, so you break the procession of brake lights, and prevent those phantom jams caused by Typical Drivers. Your change in speed was so gradual and imperceptible (sp?) that your passengers didn't even notice it. Scenario 2....
The skillful driver is driving in a single-carriageway national speed limit, not using cruise control. There is a 40mph limit ahead. The skillful driver lifts off the accelerator at exactly the right time, so that when he passes the new speed limit signs, he's lost exactly enough speed for that limit. (Advanced driving observers & examiners just love it when you perfect this...).
In both scenarios, the Typical Driver has maintained speed for longer - which requires fuel to do so - and then used the brakes when he has to.
But by the time the typical driver brakes, the skilful driver had already lifted off to adjust speed several seconds ago, and had not used any fuel during the same period.
We're only talking tiny amounts of fuel. But repeat this process over a long journey, and the skilful driver without cruise control will use less fuel than a typical driver who IS using cruise control.
On a long journey at constant speed with no speed interruptions, these situations would not occur, and there should be no difference in economy between using cruise and manual control.
HOWEVER - this type of journey rarely happens in the UK (unless you're motorway driving at 3am) and you're pretty much guaranteed to have to slow down and speed up on a fairly regular basis.
Under these circumstances, a skillful driver will use less fuel than someone using cruise control.
My Ibiza and both Passats had cruise control, and I think I can count the number of times I've used it in the UK on both hands. I've always achieved better economy and a smoother driving style without it.
TBH I think cruise control is a waste of time in this country unless you're a nocturnal motorway driver.
On the continent it's been a different story. On the autoroutes through France I've driven for well over an hour on cruise control without once having to adjust speed - partly because there is much less traffic, and partly because drivers on those roads have much better lane discipline. In these situations the car has returned mega mpg.
I think I've even bored myself now, I'm going to get a coffee
where an automatic is sensible
.. in fact they could nearly walk faster (as I used to when I was there...) And I use my brakes... oh perhaps 10 times in 25 miles at junctions/roundabouts .. and end up having to use the brakes a little harder every couple of days, and use handbrake to try and keep the 
