Have had them on two coaches for close to 100,000 miles. They do help the handling, especially in high winds and when trucks pass. I never keep track of mileage so I can't say whether they help for that or not. They do keep the rear of the coach cleaner. I had them on the sides and the top of both coaches. For the price of them, which is less than a tank of gas, they are well worth the added stability on the highway.
They were also a lot less expensive than a trac bar, which I do not have and don't see any need for now. I usually travel the speed limit which means 70-75 MPH on the highway and I have no problem with the stability of my gas coach. The first coach I had them on was a diesel.