I see a lot of movies in theaters and so far I've never walked out of a movie before. For me to even consider walking out, I'd have to hate that movie right off the bat. And while there has been and always will be some "sub-par" movies, I can sense well enough beforehand what I'd be getting myself in to and then figure if it's worth seeing in theaters or not - varies from person to person ;)

I remember one time, though, when I had the highest desire to leave during a movie. My gf at the time was obsessed with anything to do with vampires and, even though I knew well-beforehand how absurdly terrible it was going to be, she really wanted to go see it at the theater anyway... Bloodrayne was the movie. Fortunately we weren't together during the times of Twilight so I've never had to see any of those XD