I'm one of those irascible traditionalists who has been riding for 40 years without a mirror and doesn't plan on changing soon. I tend to side with those who would be damned if they have a tiny piece of breakable glass so close to their irreplaceable eyeball. BUT -

I've mounted one of those handlebar-end mirrors on my tandem. It's funny how quickly you get used to it. I'm not at the point where I'm dependent on it, nor am I ready to put one on my primary single bike, but I don't think I'll be taking it off the tandem anytime soon.

Those helmet-mounted mirrors John Forester cautions against because they might send the wrong body language to those behind. In order to catch some backward angle prior to a left turn, you might turn your head to the right. Usually it's better just to look back; this send motorists a signal, or suggests your intentions. On the track, you signify that you're going to swing off by turning your head over your right shoulder.

Digression: Having said that, I am a STRONG believer in mirrors on a car. I set mine up according to advice given by a race car driver. The outside mirrors are set outwards such that you cannot see the side of the car. Instead, you can track a vehicle as it approaches from behind in the inside mirror, then as it begins to pass, you start to see it disappear from the inside mirror, but simultaneously appear in the outside mirror. This is how you avoid blind spots. I get really annoyed when I take the car in for service, and the outside mirrors get readjusted by some ignorant mechanic!

Luis