I've used various and like others find that Take-a-Look works the best for me. As mentioned, the mirror position is stable and the mirror is pretty steady compared to a helmet mount and especially compared to a bar mounted mirror.
I can see some small potential advantage to wearing two, but I can rotate my head a bit to scan the view in the mirror or a wide range.
As for refocusing your eyes when you view something in a mirror, you're focused on the object your looking at, not the mirror. So, if you're focused in the distance (>20') in front of you and glance in the mirror as some object in the distance behind hind you, your eye's don't refocus. If the mirror is convex ("objects appear smaller") this is still true for objects greater than 20'.
Last edited by Looigi; 03-06-11 at 01:47 PM.