No matter how tight you tighten the adjustment nuts the mirror likes to move around at highway speeds. I got them for the price i ride hard and like to break mirrors so for 10 bucks cheap and easy to replace. They stand up pretty good against tree branches and scratches.
The folding mirror has saved many replacements. Yes, I have broken one - and it broke without damaging the mount or the handlebar (like it should). My friend became sold on them the first time he went down off road. They are solid and easy to adjust (ok, they will work loose if you fiddle with them while riding) as long as your wrenches aren't too thick. they are inexpensive enough that I usually carry a spare with me now.
Garbage. These are completely useless. The mirrors don't stay in place at all. As soon as I start moving the wind blows the mirrors back so I can't see a thing. I don't even have to be at highway speed for that to happen.