You can blind ennemies when fused to a shield. Also someone on youtube built logic gates with them
Outside of the puzzles they're not super useful. You can blind some things but it's not always easy to aim and weather dependent.
Use like a kick stand for motorcycle designs.
I've not and I've not seen any really cool implementations of them either
I think I avoid them because they're mildly infuriating based on the angle of reflection.
I've only used them for the respective puzzles so far. Technically you can blind enemies with them, but if you have the choice between attaching a mirror to a shield or a flame/frost/beam emitter instead, go for the latter. Mirrors don't deal damage. (Maybe they would deal damage to stals, but since the mirrors don't have any light to reflect at night ...well. Duh.)
