One of my mirror covers was cracked by an impact with an oncoming vehicle when my van was only a few weeks old. Instead of replacing the original cover, I ran an automotive PU adhesive/sealant around the damaged area and fitted silver MirrorGuard covers over it and the other mirror. I haven't suffered any other damage since and the effect of the Mirror Guards, being silver rather than black has a dramatic effect on the behaviour of oncoming drivers. Whereas before I found they would often barrel through on narrow roads even when I had stopped, they now pull over and give way quite consistently.
My assumption is that the original black mirror covers didn't register with many drivers, and they related the available with of the road simply to the van body itself, whereas with the MirrorGuards I am convinced that they think that van is even wider than is actually the case.
I'd highly recommend them, both as a method of repair and prevention of further damage. They are constructed similarly to motorcyclists helmets, with a robust outer shell and a foam lining, so the likelihood of further damage is greatly reduced.
I used the PU sealant between the Mirror Guards and the original mirror covers on both mirrors, since it also has the benefit of preventing them being pulled off by a thief, something which again is probably unlikely unless you are very unfortunate, but I have heard of it happening more than once.