How safe are helmet- or glasses-mounted mirrors? I am wanting to get a mirror for my bicycle, but the obvious mount points are already occupied (bar-end shifters, aero brake levers).  There is some appeal to the Take A Look or a helmet-mounted mirror, but I am concerned about safety. It seems that a mirror hanging out near my eye may have the propensity to be jammed into my eye or some other part of my face or head in the event of a fall.

What happens to a head-mounted mirror when the rider falls? Do they flex or break in some helpful way? Are my fears well-founded or overblown?
I would say that there's not much more concern of the mirror gouging your eye out than your glasses in the event of an accident. Ultimately I think if you did a risk/benefit assessment of having a mirror, you'd come out in the black.
For most cycling mirrors I'd say that the mounting point is going to give way before anything breaks and becomes dangerous. If you're truly concerned, have a look at this question where I tried to put together an exhaustive list of cycling mirror options.