I started riding with a helmet mirror over 10 years ago and now find it pretty much impossible to ride without one. As a matter of fact, I ride more than I walk and when I walk I constantly find myself instinctively looking up for a mirror that is not there. It really drives me batty if for some reason I ride with a different helmet that does not have a mirror mounted to it.
The mirror of choice for me is a CycleAware Reflex. I have never ridden with any other kid of mirror but I can imagine that a handlebar mounted mirror would suffer from excessive vibration.
I also wear glasses when I ride and I know there are glasses mounted mirror options, but my frames are super light and I don't like heavy glasses on top of my nose.
Thus the helmet mounted mirror is the winning choice for me. The only downside I found over the years is that I bang my helmet around and have managed to pop a couple of the CycleAware mirrors off. The mirror does have a quick release of sorts, you can pop it off of the mount, but doing this repeatedly can lead to dislocation of the mount which comes with a sticky pad from the factory for attaching it.
For me the solution came with Gorilla Glue. I don't much care about the tidiness of my hacks but for those who are concerned about such things, be warned that Gorilla Glue will foam up. I had never used it prior to this project and slathered a liberal amount on. You can easily see the result on my helmet -
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If you don't ride with a mirror I would highly recommend giving it a whirl. It is especially useful in an urban setting. I have enough to worry about with potholes, manholes, etc. and to have to look over my shoulder is just one more thing to distract me. If you do ride with a mirror and have your own advice or recommendations to pass along please chime in with comments.