My neighbor swears by his helmet mirror, but my helmet already has a big add-on visor that complicates mounting. For now, will try a Mirrycle bar-end mirror.
I had issues with handlebar mounted mirrors because I had to mount them on the ends of my handlebars so I could see past my shoulder, then they would get caught up on stuff.
The Hafny mirrors had to be fully loosened off to fold them in and out of the way to get my bike through my doorway.
It was a PITA.
My helmet mirror is working great for me now but it took me a week or two and hundreds of kilometers to stop looking down and to the left to see what was behind me.
I've got lots of room for visor clearance if I had one,..
I ended up gluing it between the helmet outer shell and the Styrofoam liner to keep it from moving around, and it looks nice and tidy.
I also have a full face helmet that I've had for a year but have yet to use.
The mirror would stick nicely to the side of this helmet too,..
I've also got a couple regular flat wire mirrors that mount to your glasses but I haven't tried them either.
I like the velcro mounted mirror.
It's slightly convex with a larger field of view, but not so much that a car (or one of those speeding pedestrians) has to be ten feet from you before you see it, so I bought two more in case I break one off in my eye.
There cheap enough to try one and see what you think before you invest in the $50 fancy version.
They stick on with the supplied velcro and are removable
Less than seven bucks, or fifty cents American money.
Helmet mirrors allow you to see up and down hills as well as around corners.
You just move your head instead of adjusting your mirror.