I have a pair mounted on one of my helmets. It seemed like a good idea at the time, but during real use they tanked. Because the mirrors are so close to your eyes 1) you have to shift your eyes way too much to look into them, PLUS you have to turn your head / helmet slightly to the side to see behind you, and 2) you also have to refocus your eyes to look behind you. All-in-all, they are not worth it IMO. I found them too distracting. I ended up using my handlebar mirrors 90% of the time.
Besides they look REAL FUNNY.
A good pair of handlebar mirrors are way better, since all you have to do is shift your eyes very slightly, and you can still see the road ahead.