The first 'Transformers' was so bad that I drove back to the store at 3am to give it back and have refused to watch any of the sequels.
To this day I've never made it all the way through 'Being John Malkovitch'. Three times friends brought it over for movie nights when it first came out, and we never made it more than about 45 minutes to an hour into it before shutting it off and either putting in something better or just drinking and BSing the night away.
However, the only movie I've ever fallen asleep in (and I'll get skinned for this) was the first 'Lord of the Rings' film. Slept away the last hour and a half, and refused to see the others in theaters. The battle scenes in the other two were good, but on the whole I didn't enjoy them- but then, I loathe the fantasy genre. Another one friends force me to watch.
But the worst for me, at least as far as disappointment went, was 'Crouching Tiger, Hidden Dragon'. After all the hype, all the awards, and all the 'OMG you have to see this movie' talk from said friends (I should know better by now), the at-the-time girlfriend and I rented it one night, and to this day I have no clue what the heck we watched. It made absolutely no sense plotwise, and I've seen Jackie Chan movies with less cheesy dialogue. I actually took it back as soon as we'd finished watching it and demanded my money back for the rental.