What is it with the fascination with mirrors? I've read many opinions, and the consensus seems to be that you can't see crap in them, and that if you do see a bandit in a rear-view mirror it's too late. Below are a couple pics from one of the newer Il-2 mods that provides mirrors for aircraft that had them. Now, before you all call me a blasphemer, inspect the images and notice their value for demonstrating the worthlessness of rear view mirrors. As an experiment, I tried to use the mirror against some AI 190s instead of turning my head diligently, and was shot down by a rear hemisphere 190 that I never saw (until my aircraft was blown to bits and I had the god's-eye view of the 190 flying through my wreckage).
This is the standard, 90 degree fov. You can see the icon of the Spit (red is friendly) about 570 meters back. The box in the lower left is some artifact of snapping a picture that has a mirror image in it, and isn't there in the game.

Here is the 30 degree fov of the same image:

The mirror is like the side view mirror in your car, "objects are closer than they appear." It encompasses a wide viewing angle. The top of the v-stab is at the middle point in the mirror (the other vertical object is the mast). So although you can scan a wide area of sky in the mirror, you still can't see crap.
Fortunately, the mirrors can be disabled with push of a button, so they do not obscure my view of my lift line.