I think pair programming can be incredibly effective, and I don't mind doing it every now and then, but a company I've interviewed with states they pair program "95% of the time" (they're a consulting shop, so they do this on client sites).
This worries me because it sounds very demanding/exhausting. I realize it increases productivity, but to do it all day, every day, may actually decrease my morale and ability to think. I wonder if they sell this idea to clients to assure them of greater productivity...