At this point, I haven't used a mirror in over two years. I had one on my bicycle when I was hit by a car (he "T" boned me trying to turn in front of me to the right while I was in a bike lane at 25 mph). The mirror I had was a handlebar mount, and would constantly vibrate on the rough roadways.
After that accident, I decided to learn more about riding, and my son got me a book (more on that later) which discouraged using mirrors. I raised my handlebars, and learned to look around. This is not a glance, but a very effective look to see what's happening around me. Not only do I see better, but I get to look the driver in the eye and signal my intent with both hand signals and body language that I'm turning, or doing something different.
If I ever get a recumbant, I will begin using mirrors again. I may get one for my glasses or helmet, but it won't substitute for me looking around and being aware of traffic, especially when I'm going to do something.