I've been back and forth on mirrors. Rode for years without one, but I'm really good at turning my head and checking the road often without veering, and I've passed many a rider who had mirrors but weren't paying attention anyway. Then I won a handlebar mirror, it lay around in my spare parts box for 6 months or so, I put in on on a whim, got really distracted, and kept turning around to check the road anyway from old habit. But after a while I got used to the split-focus thing and enjoyed the ease of being able to keep tabs on my back at all times. Especially useful when I got another cyclist on my wheel, actually.
Then I took a tumble, broke the mirror, and freaked out a little at feeling "blind". Planned to go get a new one, but within a week I'd got used to turning around again, and I still haven't replaced it.
I feel they're a great idea and a good supplement (and it spares your neck and back a bit) but looking behind you properly is a basic skill that needs to be there anyway.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett