Like some of the others, I don't generally walk out on a movie because I paid my good money to see it (almost 11 dollars here in NYC), so I just force myself to sit and eat it.
There have been 3 exceptions though in which I had to make like a tree and get out of there. I absolutely just couldn't stand it any longer, and they were really shyte low budget movies anyway. One was Cyborg with Jean Claude Van Damme (yeah, yeah, I know), Jason Takes Manhattan, (I swore to God himself that I'd never watch another Jason movie again, ever....), and Stallone's Lock Up.