I use one of the (Take-a-look?) mirrors that mount on my glasses. I ride as much solo (and much in traffic) as I do in groups where it may be less helpful and I find it handy for my constant scan of what's going on around me. I mean, when I drive a car, I check the mirrors every few seconds, so why the hell shouldn't I when I'm riding something WITHOUT a steel cage around me on the same roads? I have referred to it as my dork flag but ultimately, I'm not going to get too upset that I may look as silly to someone else as that person may happen to look to me (know what I mean?)...it doesn't matter. My observations on the mirror I use are:

* easy to add it to a quick eye scan motion which for me is simpler than turning my head...and safer, I'd think...though quite fractionally so
* with it mounted to my glasses vs handlebar or helmet the position relative to my eye is constant, so I get a good rear view scan in any position on the bike (really)
* In sketchy traffic situations, the mirror has been very helpful in helping me know when & how to communicate with approaching drivers from behind (sometimes to appropriately claim a safe part of the lane, others to get the hell out of the way because grandpa or bubba is barreling up & talking on the cel phone).
* I'm so used to using it that now I feel like I'm missing some critical information (I guess, like driving a car with no mirrors) when I ride without it.

It's not essential, but for me it's far more helpful than it is detrimental to my reputation for looking like a dork to an elite few riders!