@Stephan,..
I remember when when I first got my helmet mirror, and you suggested that it could poke my eye out if I crashed, but I always wear goggles and the mirror will break away from the helmet quite easily in the event of a crash.
You also suggested that it would affect my focus.
I thought that you were referring to my eyes trying to adjust their focus to see what's in the mirror, but the mirror is far enough from my eyes that my eyes aren't constantly refocusing.
I realized later that you were probably referring to my mental focus and taking my mind off the road in front of me while I'm looking at what's behind me.
That was actually a problem at first, and I found myself staring at the mirror for quite a while trying to aim the mirror by moving my head around.
I've gotten a lot more used to using the mirror now and I do a split second check of the mirror about twice a minute.
Only my eyes move to check the mirror, so my head is always facing forward.
My peripheral vision takes over while I'm checking the mirror, and without looking down to the handlebars to see the mirror, my peripheral vision works better.