I use the Third Eye that mounts on sunglasses. Adjusts easily and I find it simple to use. The only drawback is that the attachment tends to break. So I tape it to the temple of my sunglasses. That has worked like a charm for multiple long tours.

I feel the mirror is possibly my most indispensible piece of equipment. It has saved my butt a few times when cars were approaching too closely. A helmet may have helped protect my head if a car hit me, but the mirror prevented me from gettting run over in the first place. If I had to choose between using a helmet and a mirror, I'd opt for the mirror.

Mostly, I like it because I can see what's happening behind me. I can tell if a car going is going to give me 6 feet of space or 6 inches and I can ride accordingly. I can see the car's blinkers and know if he is going to pass or turn. I can tell if I need to ride on the last 2" of pavement or if I can ride out in the road a little bit. And I actually once foiled a thieve in Buenos Aires who was tryng to grab something off the back of my bike at a stoplight.

The biggest problem is that at the begining of winter I instinctively look for the mirror while skiing.