I've tried those too and didn't like them. The latest was take-a-look, which would be OK for some but I didn't like. The helmet mount was dodgy and my glasses are very thing wire-rimmed flexons, the mirror barely holds.
I just put on a CycleAware Reflex, and it's the best. The first day out, after a couple of miles, I realized that I had been checking behind me with the mirror totally without conscious effort. Of course, proper adjustment is critical, but once that's there, it's a great mirror.
I'll probably lose the bar-end now.