I wouldn't normally walk out of a movie theatre but as it was empty at the time I walked out on Pan's Labyrinth... too violent. I would have loved to have walked out on Sin City and Pearl Harbour... just bored. Avatar tested my will too - but the 'pretty pictures' kept me there.
At home, I turned The Postman off at about the 1/2 way point - and I LOVE Kevin Costner. Snorefest.
When I watch LOTR at home I just fastforward anything to do with Hobbits and Ents. Great movie(s) - without them ;-)