I like the Third Eye helmet mirrors. I attach them to my helmets, not to my glasses, and they have been wonderful.
Helmet mirrors take a week or two to get used to- them seem very odd at first but then you can get to love them. Unfortunately, lots of folks try them for a day and then toss them.
I wouldn't ride without one now, it'd be like driving a car without a rear view mirror. Sure I do a quick check with my head too, just like driving a car....but the mirror has made me safer on many occasions.