I'm such a clutz. I'm sure a mirror wouldn't last a week in my commuting gear. If it was attatched to my helmet I'd break it the first time I went to wipe sweat off my forhead or at the latest when I got my destination and dropped my helmet while taking it off because my hands don't always do what I tell them to. If it was a handle bar mount it would be toast before I even got going, at least twice a week my bike falls down while I'm gearing up.
In short, if someone were to give me one, I'd use it until it broke, but I have better things to throw away my money on, like beer. So I ride slow, listen carefully, check twice at least before any turn or merge, and if worst comes to worst I stop on the right hand side and wait for traffic to clear to do my left turn or what have you.