I feel cheated out of the extra time I spend on something I've decided is crap. At the cinema it's more rare that I walk out of films, often one has to silently negotiate with the person one went with. I've had a few occasions though were we both loathed the film and were like "if only you'd said something, I would have left too". At home I skip lots of films I can't get into.
There was one film I gave up on after twenty minutes and then returned to after I saw the film-makers following film, which I liked. The film was the "mumble core" psycho thriller A Horrible Way to Die and the follow up was the satirical home invasion film You're Next. When I stuck it out to the end the second time I really liked A Horrible Way to Die, even if initially the way it's made struck me as confusing and off-putting. So occasionally I get it wrong, but I think it's rare and not worth wasting time on all the films where I don't enjoy myself.