Well I remember seeing the ads for The Matrix, and seeing Keanu Reeves star in it and refusing to see it. Ended up renting it and enjoyed it.
As a wee lad, I thought Braveheart was going to be a cheesy romantic epic. Turns it it was the war movie that redefined war movies.
Grandma's Boy was going to be awful... but then I saw it and it wasn't anything like I expected, and I laughed until I could barely breathe.