yes, i carry one in my get-home-bag.
there's also one in every vehicle.
i've tried the reinforced glass, but i switched to metal.
they've only been used as an actual mirror (no signaling).
i've practiced with them, and they work well, but.....
someone showed me that they are not that great.
from my fading memory, these were his points:
1. ~50% of 24 hours are dark.
2. 50% of daylight hours (in NC) are too cloudy/foggy/etc.
3. 50% of the sky is blocked by trees/buildings/mountains.
there were others, but you get the idea.
do the math, and get ~5% possibility of right time and right place.
i still carry them.