Wind deflectors - has anyone used one and do they work

I had several friends in the business of delivering RVs to dealers. They all at one time or another used deflectors and over a period of time most gave them up. They didn't do much while towing except move the bug line up. They hurt the mileage while not towing which for most of us would be the majority of time and were a real pain to put on and take off and store. One did keep his because as he said "it made his truck look cool".