Another vote for the glasses mounted Take-a-Look mirror. It is, by far, the best mirror I've tried (and I've tried a bunch of different handlebar and glasses mounted models).

I ride mostly on semi-rural roads, and the mirror allows me to easily keep an eye out behind me without having to twist around. With the mirror, I can check behind me a lot more frequently than I could be turning around, and I can see cars approaching long before I can hear them.

We could drive our cars without rear view mirrors, but why would we? A good mirror, like the Take-a-Look, increases "situational awareness"...and that's a good thing.