"The debate over mirrors thankfully is not quite so heated as helmets, earphones, bike lanes and shared paths." or shaved legs

I just bought a mirror at the LBS. The guy there was quite honest "You're wasting your money". After having set up said mirror and then seeing how useless it was I removed it.

The only one I think that would be practical is the Rhode Gear Oval Mirror. Even it wouldn't be brilliant all the time. I can see mine getting knocked off every time I put the bike away etc., etc.