I bought a mirror but it is worse than useless. The clue I missed was, while in the LBS and the salesman was telling me that people who had bought this thing really liked it, and I was looking at it - saying to myself 'I can't imagine from the way this has to be mounted that it can possibly be in a useful position for rear viewing'. But I bought it anyway.

So, I was able to see cars coming from behind by - first detecting them aurally then quickly readjusting the mirror so that if I looked between (in the mirror) my arm and my torso - and had the elevation just right - I could get a quick look.

It seems to me that this could add excitement to the ride, especially for the ones you don't hear until they are almost on you.