I use the "Third Eye" mirror and really like it. It took a little work to get it fitted into the helmet but it works well. Yes, you do occassionally have to adjust it. Indeed, I may adjust it several times as I change my riding position. I had never used one until about a year ago. Now I would feel uncomfortable without it.
However this thread, as much as anything, seems to point out to me the inherent risks of seeing how fast gravity can accelerate 20 pounds of aluminum and carbon fiber carrying 200+/- pounds of flesh and bone on two narrow wheels that are inherently unstable when traveling in anything other than a straight line on anything other than a smooth surface.