I tried this and it absolutely did not help. In a "normal" situation where your desk faces the rest of the room, you would become aware (in an "ambient" way) o

I tried this and it absolutely did not help. In a "normal" situation where your desk faces the rest of the room, you would become aware (in an "ambient" way) of people wandering into your field of view. But with your back to the room/hallway/whatever, you need to explicitly scan the mirror (sort of like polling, in computer terms) with frequency to know if somebody is approaching. It's like trying to work while also focused on a video playing in a desktop window or something. Of course, as with anything, some people are not bothered by this. (And, of course, some of those people are fooling themselves into thinking there's zero impact on their focus/productivity)