What, I'd kill you to ring your bell? I have a mirror, and occasionally am passed by someone who I didn't notice back there. Inevitably it happens in heavy traffic when all my attention is on the cars in front of me and the mirror is aimed at the car traffic coming up behind and to the left of me. If someone on a bike turns onto my street and passes me within a block of doing so I may not spot them behind me, and I have to admit it often startles me and irks me. Not that I'd say anything aloud though.

Back on topic: I have a Safe Zone mirror. This mounts to your helmet, has a very adjustable arm to position it just where you want it, and has a nice (biggish) sized lens. I highly recommend this mirror, very easy to adjust when you need to, it shows no signs of wear and tear after nearly two years of daily use, and I really appreciate the field of vision you get from the large mirror close to your face.

I haven't tried the smaller helmet or glasses mounted mirrors, but I was concerned my aging eyes would have a hard time switching from the road ahead to a very small mirror. Also, although I always where glasses or prescription sunglasses, I didn't like the idea of messing around with putting and taking the mirror off of my glasses every time I rode. I tried handle bar mirrors, lots of people say that they work fine for them, but most of the roads I travel over are so rough that I had a hard time focusing on the handlebar mirror. With an impact every 5' or so from seams in the pavement or just plain rough asphalt the mirror might as well have been vibrating. It probably didn't help that I ride pretty upright and so the mirror seemed pretty far away too.

The only time I'd consider riding without my mirror is mountain biking. Although I rode for years without one, now that I have ridden with one I consider a mirror essential.