I always ride with a helmet-mounted mirror. I like being able to check for the next wave of traffic; if it looks particularly bad on a busy stretch of road, I can pull over and wait. And since I have some neck issues, I don't like turning my head around just to see what's behind me. Like Jim from Boston, I feel like I drift slightly if I turn my neck too far, and I like the option of doing a quick mirror-check.

Seriously, until they develop an on-bike radar system, whatever works for you is fine. I like a mirror, but I also like lights, bike computers, a place to put my phone, etc. If you feel better riding with a mirror, figure out if you want a bike-mounted one or a helmet or glass-mounted version. See how you like it.