I've never walked out of the cinema, nor would I ever, unless I had urgent business to attend to that just came up.
It's so stupid.
I generally don't go and see crap films though, so it's fine, and when I do (Wolverine), I know they'll be rubbish, but still plan on seeing them. (Or am forced to see them by family - Bratz, Make It Happen etc)
Not seeing all a film and deciding it's shit makes no sense. It's like looking at half a painting then judging it. Your judgement may end up being right, but I don't believe it's entirely fair to the art itself.