I tried a helmet mirror and discovered a problem with them that no company mentions: they don't work if you ride in an aggressive position with a big right-swing messenger (strap over the left shoulder) bag. The bag completely and utterly blocks any view you have over your left shoulder. This would also apply to any mirror that attaches to the eyeglasses; there just isn't any line of sight from the left side of your head behind you.

I might try a handlebar mirror for my drops someday. Supposedly this is the best one, from what research I did: http://www.aspirevelotech.com/Mercha...ry_Code=MIRROR