It took me about 2 hours of riding to adapt. But for a couple weeks I still squinted one eye to see into it (to get rid of the overlapping images).

$8 sounds reasonable. I think mine cost $14 plus s&h.

I only use the mirror to ascertain that there IS a car behind me. If I think there is NOT a car, I confirm it by scanning. I think there may be a blind spot with a mirror. It's also really useful for keeping tabs on riders behind me, if I'm riding with my daughter or someone.