I haven't had the problems with my bar-end Mirrcycle mirror that others have reported with their handlebar mirrors - no extreme shaking, I can clearly make out what vehicles are coming up from behind, etc. And yes, the mirror has proved useful for detecting other cyclists silently coming up behind me. I'm sure some of them feel sheepish when I suddenly say to the almost-drafter behind me "Go ahead" or "Hey, I'm going to turn right/left here" when I'm descending and have to use my brakes (thus hand signaling is not possible).

That all said, I'm probably going to lose the Mirrcycle when my swept handlebar is replaced with a trekking bar, because the bar ends will be facing inward instead of outward. I'm probably going to order this helmet mirror as a replacement - it's not glued to the helmet like other helmet mirrors, which I quite like.

HubBub Helmet Mirror | HubBub Custom Online