Mounted on my glasses it has to be far enough from my head to see over my shoulder, but in that position it's outside the area of focus for the glasses, so cyclists behind me are just a blur. Works fine with contacts or plain sunglasses.
Granted it looks a little dorky, but now that I have a mirror I don't feel complete if I am on my bike without one.
I prefer the head mounted mirror to the handle bar mounted variety, less vibration, easier to control field of view.
It is easy to mount on a visor, but not on a helmet with out a visor. Found an attachment device the works pretty well. Once mounted in a good location it is easy to a line the mirror for looking at approaching traffic. It is very light weight and good visual field.
I am car-free and wear this mirror attached to my helmet visor. It allows me to keep track of what's going on behind me without constantly turning my head around. While no substitute for scanning by turning my ahead around before merging into traffic or a left turn lane, the mirror saves my time by allowing me to monitor traffic and pick a better time to make my move. I see many, many experienced riders wearing these, including long distance tandem tourers wearing one on each side.
I love the elegant infinitely adjustable angles. It was easy for me to set up, and I find the mirror an extremely helpful tool without being a distraction. Super light and strong.
Longer than some that I have seen. Works well, but end that attaches mirror detaches rather easily. My frames have triangular shaped temples and these would work much better with flat ones.
I attached the mirror to my helmet visor. Took awhile to adjust the mirror and make myself look out of my left eye. Once that was mastered, the only thing is at each ride I had to readjust the mirror as the helmet gets bumped around - taking in on/off and stored. These are not big issues - we all adjust.
This mirror fits securely on my glasses and is fully adjustable. The size is enough that I have great visibility. I have tossed the other mirrors that I own and use only this now.
Attach the rearview mirror to the helmetsunglasses. Twist and turn to adjust for maximum coverage and off you go. Not only that you can see how the paceline is conforming behind you but you will also see a car approaching from the back. Safety above all else!