I recently put one of those on my left-side dropbar end. It's way better than nothing, but I find it's so far away from eyes that the view from it is tiny. I also can only see it when I'm riding the hoods, since if I happen to be riding the tops my arm blocks it. I don't really ride the drops so I don't know if my arm would block it there too, but they probably will. Due to its shaping giving it that "objects in mirror are closer than they appear" effect, and my eyesight being something less than perfect, this mirror is good for a "ok, I'm pretty sure a car is close enough that I gotta look over my shoulder now to know if it's safe to change lanes" and that's about it.

I haven't got a helmet-mounted mirror yet, but if I really want to see what's behind me better, it's probably going to have to be something like that.