Gotta agree with this. I was convinced my helmet mirror wasn't working for me because I've had LASIK and my vision is corrected for far distance. I kept it on, though, because I could catch headlights at dusk. It took me a couple of weeks but all of a sudden, I glanced in the mirror and realized that I could see behind me clear as day. I don't know if I just got used to it or the angle was slightly changed but now that I KNOW I can see behind me with it, I adjust until I get the right angle every time.

I also have a bar end mirror on the left of my flat bars. Between the two mirrors, I've got my rear covered!