I haven't every walked out of a movie because of the movie; I've had to leave a couple of times due to outside stuff (including a medical emergency, yay) but not from the quality of the flick. Part of that is that I'm picky about what I'll watch in the theatre, and part of that is I don't want to abandon a date/group. (And I'm frickin' cheap, come to think of it.)
Still, there's been a few that tempted me; that D&D movie (Jeremy Irons, oh, man, why?), Robocop 3, Alien 3... and the closest I've come is getting up repeatedly during Deep Star Six (a hasty Abyss clone, for those who don't know) to loiter at the concession counter. Man, I got so sick from ODing on popcorn from that...
-- Steve