I have used signal mirrors a fair amount. They are fabulous, at least in the western US, with lots of sunlight. I have broken at least one glass mirror, but it was still usable, since the interior cementing layer still held everything together (and I had an extra cutting edge!) I just looked at a plastic mirror I had stashed away in a kit, and it was quite a bit hazy, just from sitting around.

Nowadays, I don't routinely carry a dedicated signal mirror, but I make sure that my compass is the mirrored type. The surface area of the compass I use is about the same as some of the smaller dedicated signal mirrors.

I am not too concerned about the somewhat lessened accuracy of aiming of a compass mirror. If I am using one of those, I will be painting the sky, the hills, and the whole visible universe to get someone's attention. I have used the compass mirror for real a few times, and it got the job done.