I used to have a helmet mirror, but I was a little nervous about what would happen should I crash and have the mirror gouge my face. Then the mirror broke so I replaced it with a bar-end mirror on my drop bar. But that vibrates so much relative to my eye that I find it almost useless. I"m thinking about going back to a mirror attached to my head, but this time one of the light-weight mirrors that clips to my glasses, rather than the more bulky type that attaches to my helmet.

They are very useful, though....