I've enjoyed reading the lively discussion.

I don't use a mirror. I haven't used one since I was 10 on my 3-speed banana seat chopper, and I can't recall how I liked it. I may try it one of these days,. As already stated a few times in this thread, it is most important to be alert and aware of your surroundings, much like driving a car. A mirror may or may not help in this regard. I've read some mixed opinions.

I leave for work pretty early so there's very little traffic on the road. When there is an approaching car behind me I can hear it coming, and as soon as I hear it I take a quick look to see how many are in line. I usually take another couple of peeks before the car catches up to me. I suppose if you have neck or back issues then this is not an option. But looking back also alerts the driver that you see him.

My commute home is a bit busier, and though you can still hear the traffic, it does little to alert you of their proximity to you. I can see how a mirror might be useful here.

In any case, your ears is another 'tool' that you can use to your advantage, which amazes me how many casual cyclists I see wearing earphones.