I used one until I got hit. After seeing the vehicle in the mirror, I moved over as far to the right as I could (I'm usually over as far to the right as safely as I can be regardless of vehicles on the road). I don't know if he drifted over too far, or if I couldn't gauge just how close he was riding to the shoulder, but the point is that short of dismounting and climbing into the ditch immediately off the shoulder, there is nothing more I could have done to avoid being hit.
I do all the same things now as far as defensive riding goes, but instead of a mirror I use a superbly bright light, visible in daytime. I also plan my routes so that I avoid times and/or roads where I'll be riding into the Sun.