I too have used the cruise for many years while hauling... it makes for a smooth ride and saves a bit of fuel.
However, it depends on the traffic and the terrain. If I am going through hills, I do not use it, for the previous reason mentioned. If it 'falls behind' it will kick on hard and I dont want to do that to the horses back there by having the sudden acceleration.
Also if it's in town, or denser traffic, I probably wont use it because the speed will vary too much as you are going along, and there is no sense in setting it.