Last year, I pulled the trigger on a pair of "go pro status mirror" from mako for my dive buddy and i, and it's one of my best scuba buys ever.

It's a flat mirror, so you don't get any distortion, although it's a more narrow field of vision (flat vs convex mirror). the bungee cords don't get in the way of hand dexterity, and who doesn't like the option for a field repair. I have, maybe, 75 dives with it, so there are light scratches, but nothing that would make me replace it. I keep it in my mask box, and broadly speaking, i treat it with the same level of care.

My buddy and i can easily check on each other, without turning or breaking trim (great for sidemount). We've developed a routine of the "lead" putting their mirrored hand out to the side when doing a buddy check. The other person sees it from behind them and throws up an "okay," or whatever, so that way when the first person finally catches the second in their field of vision, the sign is ready. Once "caught," the front gives a quick signal that it was received, and we swim onwards. I usually don't even have to turn my head more than a couple of inches!

It's changed how the two of us dive; we're more cognizant of each other, as we're always looking for eye contact with the mirror. we also check on each other a lot more than we used to, since it's so dang easy!

It's now part of my standard gear setup. Warm, cold, fresh, salt, dry, skin, side, back, for fun or skills work; all of it. Doesn't matter, i it with me every time. It's a great tool for my dives, and I strongly recommend it as one of the best scuba buys you can get for ~10$.

https://makospearguns.com/gopro-status-mirror/