I cycled for over 25 years without a mirror. I was finally converted when I fitted one for cycling in Greece. Whereas I agree that nothing beats looking over your shoulder, especially in town, mirrors are great for seeing what's coming from a long way. I can see cars coming a long time before I hear them, and when they do eventually get close, I find it reassuring to see them putting on their indicators and moving out.
The mirror I had last year was a Cat Eye one - and I put it at the bottom of my drop handlebars. Here is a photo I put on my blog:
But the trouble with this mirror is that when carrying back paniers, my view behind was blocked. It eventually got ripped off in a train, and so I've bought a new one. It looks like this:
You'll find it
here
I haven't used it much, yet, but so far I'm very pleased with it. It only vibrates a little bit and gives excellent visibility. It does need to be tightened very hard, though, and although it folds away, it does stick out a bit when I put the bike away in the garage.