NEVER BREAK
to turn it off unless you have to.
One of my pet hates, you see people bombing along with the cruise on and break to turn it off, scares the drivers behind so they slam on (also because people drive too close). Just leads to accidents because you get people who are chilled driving along and all of a sudden they panic when they see break lights and get rear ended. Seen this so much on the motorway, rear ending equates to a high percentage of accidents.
The way I use the cruise is to set it to about 75mph (you wont be done and can overtake slower moving traffic) always leave 4 car lengths, leaving loads of room to overtake, to accelerate just press the peddle, to slow down just turn the cruise off by pushing it upward. You can speed up by pushing it away from you and slow by pulling it toward you. If you are gaining on people then just turn it off using the leaver and act accordingly.
So many people don't use it properly and accidents happen, if you feel like your being taken away with it just turn it off, the car slows very quickly without the use of the break. Half the problem with people on the motorway, use the car to break, not the actual break, unless it is necessary. Breaking not only can be dangerous but excess ware, higher fuel consumption etc.
If more people took the extended driver training (pass plus) which includes motorways they would know this!!!
Sorry rant over.
In terms of fuel consumption it does use slightly more because when going down hill the car slows using the engine instead of say taking your foot off the accelerator slightly, but its not a lot depends what the roads are like.