Yes, a few times. Typically, it's for one of the following reasons:
1) I feel sick, or someone who I'm with is. No sense puking in my popcorn, eh? It sucks, but it has happened.
2) Technical glitch. I've left because of lack of sound, fuzzy screen, and a projector that ran 50% too slow (I'm guessing 12/fps, which is noticable). I usually make alot of noise, and get the next show free.
3) Truely bad movie. This has only happened once, and honestly I don't remember the movie, it was that bad.
Oh, and since we're on the topic, and it's funny as hell, my friends were forced to leave a theater once. Nothing that they did, either. It was during Sherlock Holmes, and (vague spoiler!) during one scene, the main characters count down from 3. "3...2...1..." and then the fire alarm went off, the film immediately stops, and everyone is rushed out of the theater. Apparently, some numbnuts drove his pickup into the ticket counter.