I used to ride with a helmet mounted mirror. I tried a couple of different types but I could never get them adjusted so that I could see very much without moving my head around a lot. Even then I found it difficult to figure out where exactly whatever I saw was in relation to me. The mirrors are small and it's tough to get the big picture of what's going on behind you.

All that time I was moving my head and looking in the mirror, I wasn't looking forward. It was almost more of a distraction. After the last one broke off I gave up and just started turning my head all the way back to look. I can do it now without swerving.

I'd consider trying a mirror again but it's not a high priority.