These helmet mirrors are popular in the States for some reason. I borrowed one once and it left me a nervous wreck. You do see traffic behind you that I would have preferred to ignore or at least be aware of without actually disturbing your ride. I spent far too much time checking behind me and not enough just riding.
I also thought if you caught a branch with your helmet or fell off the mirror and it's leg was going straight into your eye!
I also thought if you caught a branch with your helmet or fell off the mirror and it's leg was going straight into your eye!