Depends how good a driver you are. For most cruise control will save you money as you wont accidentally be speeding up and slowing down burining fuel, but it will burn more fuel around hills as it will slow you down them and then use throttle to keep speed going up them. If you are a good driver you can save more fuel by slightly increasing speed down the hill and use less throttle up it (and thus slowing down ever so slightly by the top). But you have to have good throttle control to achieve this properly, which most numpties dont have On long drives, cruise control will save you fuel by holding a constant speed and roughly constant throttle, so you arent speeding up and slowing down all the time.
Good tip on following lorries through traffic, I do this all the time, it saves time and fuel. Outside lane is the worst place to be, they leave it til the last minute to stop, slam on the brakes, as soon as it moves, they boot it again, then slam on the brakes If you try to leave space to balance this out some sod in the middle lane cuts you up I have learnt the art of spotting traffic stopping, nipping to the inside lane to undercut it all, then when it gets going again head to the outside lane!