I've never walked out of a theater but I'm sure I've turned movies off before. Many times, even if the movie is terrible, I try to wait and see it through, trying to give it a chance to redeem itself. Usually, it fails.
Examples are Watchmen, Dragon Wars, and The Rottweiler.
The only movie I can think of that I shut off recently was Inglorious Basterds. Now, just a second. It wasn't bad but it wasn't holding my attention enough to keep it going either. It was running on for too long, it was getting late, I had school the next morning, and I had unfinished homework. I didn't think the movie was good enough for me to not do my homework because of it. -shrug-
But almost every other movie, I try to let it redeem itself until the end and I would've done the same for Basterds if not for the homework.
EDIT: Thought of two others!
Curious Case of Benjamin Button. Too slow, too boring, and that was an hour and a half in. I fell asleep on it twice in that time.
S. Darko. Donnie Darko was really good but S. Darko was total trash. It was like Butterfly Effect meets Final Destination and it made absolutely no sense. >.>