I made the bar end mirror shown below to test out whether I'd like this style of mirror. I just sanded down a dowel, cut it at the right angle and glued on a spot mirror from the auto parts store.
The verdict: The general style is OK, but the mirror I made is too convex. It's OK for cars that are close, but a car that is 100 yards away (significant when I ride on the highway) is microscopic. If I could get a less-convex mirror, I would use this instead off spending $20 on a Sprintech or Mirrcycle.
I'm hoping that a commercial bar end mirror will be less convex.