I'm using the Take a Look metal kind, attached to my sunglasses. I'd rather have one stuck to my helmet, but the visor is in the way. And I like the visor, so I'll have to experiment more with the stick-on variety.

I used to hate having a mirror stuck out in my field of vision, but I finally got used to it, and learned how to turn my head to get the right view. It's really helping with my nervousness being around cars. I have bifocals, but in "rear-view", you're using your "distance" prescription. My sunglasses have my long-distance prescription, and it works well enough. As long as I can still read my bike computer, I'm happy.