Delixe's thread on games you were surprised to love seems to be doing well, so I thought we might have a similar thread for movies.
The movie I have ever been the most pleasantly surprised by (is that even remotely correctly phrased?), is Demolition Man. I'd seen the trailer and hated it. I wasn't a fan of Sylvester Stallone, and the trailer made the movie look like another completely mindless Hollywood action flick. A couple of my classmates dragged me to see the movie anyway, and I went along just to be social. Boy, was I surprised! Never has a trailer been less representative of the movie it was supposed to showcase. Every trace of the movie's fantastic humor has been surgically removed from that trailer.
The only other trailer I can recall that was even remotely as bad, was the trailer for Leaving Las Vegas, which made the movie look like a comedy. That, however, made for a very UNpleasant surprise when I watched the movie itself.

Polar Express and the last Rocky.

The Fall. Hadn't heard of this movie endorsed by Spike Jonze and David Fincher, yet from the director of terrible The Cell, and had no idea what to expect.
The title sequence alone is pretty awesome for lovers of cinematography like what I am.
The plot itself is a little old, but the actors, cinematography and execution in general make it a great experience.

Manfish! My sister and I taped it late one night assuming it'd be a sad monster movie. We were really disappointed for the first 10 minutes and then we couldn't stop laughing at how bad it was

Death Race... Not because it was a good movie but it tried so hard to be serious it turned out to be hilarious. Every line in the movie is a one liner, there are plot holes everywhere, the acting is horrible, and me and my friends laughed so hard...

The Jason Bourne trilogy. I heard about the first movie at its release, but to me it seemed like another James Bond-like movie and - as I'm not fan of James Bond (each time one is on TV, I watch it, try to like it, but fail) - I weren't interested by it.
Then I saw Bourne Identity on DVD and like it, good character, good story, strong action scenes, etc... well it seemed to me it was a modern and realistic Bond movie and I liked that.

Inglorious Basterds
I was a huge Tarantino fan back in the 90's. Then I thought the Kills Bills were just ok, and I thought Death Proof sucked ass .
I went to see IB on opening night, not really expecting much. The only reason I went on opening night was because I had just seen District 9 that night, and we were at the theater already. It was more of a " we have nothing better to do tonight we might as well watch another movie" type thing than " OMG I wanna see it right now so bad!".
Anyways I loved it. The thing I was most happy about was that it showed the return of genuinely funny Tarantino. And the movie gets intense when it's not trying to be funny

James Cameron's Avatar. I went in expecting a good-looking, mediocre movie, but it's ended up becoming one of my favorites.

Ginger Snaps. The Canadian horror movie did an excellent job on portraying lycanthropy as the menstrual cycle for the lead characters. That, and the effects were so cheesy it was good. Oh, and I have a thing for Goth girls.

Galaxy Quest. We rented it as a lark, thinking it might be funny in a kid's-movie kind of way, what with Tim Allen, Tony Shaloub, Jed Rees, and a bunch of others known for not-so-serious film and TV work, and it turned out to be a really fun send-up of Star Trek TOS. It mimicked / spoofed Star Trek well enough that when we watched the new Star Trek film we both laughed a few times at stuff that we saw first in Galaxy Quest.
Along those lines, we also had a good time watching Fan Boys.

Silent Hill the movie.
Was expecting it to be crap but turned out to be one of my favorite horror movies.
It has the scare factor, it was closed to its videogame counterpart and best of all, there was some philosophy behind it.

That is probably because Satan didn't make it.

I thought it was a good movie, but the end fight was out of character for the movie and hurt its appeal.

I must admit that I thought it was a completely shitty movie. It's the only movie I've ever seen that screamed "console game!" at me. But then I've never played any of the Silent Hill games, except "The Room" for about half an hour.