Sometimes, and I don't think it makes me that much safer. Certainly, I have never "hit the ditch" when a car came up behind me, and I've never been run over from behind either.

Ultimately, it's really hard to judge the speed of a car behind you in a bicycle mirror, and even if you could -- 99+% of the time even if they are coming at you at a high rate of speed, they then slow down or change lanes so they don't hit you. By the time you know that they're going to hit you -- it's too late to do anything about it, and if you try to do something about it before that point, you're going to be throwing yourself off the road more than ninety-nine times for every one time that they'd actually hit you.

A mirror saves me some neck strain, and lets me be more aware of things like a cop that might be following me (follow the law to the letter!) by making it easier to check behind me -- but I don't see it as doing anything to protect me from crashes, including overtaking crashes, whatsoever.