My local Audi garage fitted sets to their loan cars, presumably as most people driving other people's cars either don't give a frig, or can't park a car next to a kerb.
Within a matter of weeks they were damaged, ripped from the alloy wheel, and heavier impacts still resulted in damage to the alloy rims. The other aspect is that they look terrible, making the tyre look a lot narrower than it is in relation to the rim.
My personal thought is that anyone who has pride in their car and is careful how they park shouldn't need such a device, which isn't foolproof anyway. My last Omega Elite that I owned for over 11 years didn't have a mark on the alloys in that time purely from being careful when parking. If anyone has a reasonable amount of pride in their Omega, then surely they shouldn't need these devices......it's like putting the plastic trims on the edge of the door to prevent chips - unsightly and unnecessary if you care for your car.