I had them
Had them on the car when I first purchased it, and yes, they do work.
My car had wide tires when I bought it, and the fronts stuck out just enough to throw road gravel that would hit the back fenders. I found out that these plastic protectors were doing their job soon after I took them off (unfortunately).
I've since bought new tires and wheels and I've been meaning to put a new set of protectors back on the car. Thanks for the reminder, as I just washed and waxed the car this Saturday before putting it away. The car has to be cleaned before applying the protectors (obviously).