As far as films I actually walked out on, there've only been a few. One was Broadway Danny Rose. I can't remember exactly why, as I never wound up watching it all, but I do remember standing up about thirty minutes into it and going "What the FUCK?!?" And I actually liked Woody Allen back when he was good.
One that I almost walked out on was The Passion of the Christ. My gf at the time actually did walk out, but I stayed 'cuz my mom went with us and I didn't want to leave her there alone. Plus, it was like a train wreck that you just couldn't turn away from. It was like "Ok, Jesus got the shit kicked out him...let's show that for three hours." Wow.
Another one I left was Reds. I was younger, went with mom, and about an hour into it I said "I gotta get out of here, this sucks. I'm going to the other theater and sneaking into Raiders of the Lost Ark." I'd already seen it three times...watched it all, then came back to the other theater and Reds was still going.
Then I got pizza.