Mirror, Mirror on the Bike...
The pro bicycle mirror rhetoric is that they allow you to monitor the traffic behind you without having to turn around. The anti bicycle mirror rhetoric is that they are distracting, and that using them can lead to misperceptions of traffic proximity. Furthermore, those who do use mirrors are divided on whether they ought to be handlebar-mounted or head-mounted. Here is a post from Alan on ecovelo that explains why he uses bicycle mirrors. And here is a thread on bikeforums where members explain why they believe bicycle mirrors are dangerous, as other members argue with them.
If you look at pictures of my bicycles, it is obvious that I don't use mirrors. But I am not rabidly anti-mirror either. I just find that they are not for me. They do distract me. And they do interfere with my sense of connectedness to my environment - a connectedness that is the very reason I find cycling in traffic easier than driving a car in traffic. For what it's worth, this is also why I cannot relate to arguments that compare bicycle mirrors to car mirrors. In a car I am closed off from the road and my view is blocked in almost all directions. Also, turning around is tricky, because of the way the interior of the car is structured. On a bicycle, my view of the road is unobstructed and turning to look over my shoulder is much easier. This is why I find mirrors in a car necessary, while I find mirrors on a bike not only unnecessary, but counterproductive. Your experience may differ.
If you are trying to decide whether a bicycle mirror is a good idea, why not buy an inexpensive one and give it a try on a quiet road? That way you will be able to determine how you respond to it and whether you find it beneficial or detrimental. I can't recommend any particular mirror, since I don't use them. I've heard good things about the Zephal Spy Mirror and the German mirror sold by Rivendell, but that's all I know. If you have opinions or recommendations to contribute, that would be most welcome.