My wife and I went to see Borat this week-end and I thought it was amusing but not as funny as I have heard. Talking about it afterwards and referring to specific scenes the movie was even funnier.
This reminded me of Monty Python and the Holy Grail. When I first saw it I thought it was amusing, but not over the top great. But then when you talk about that movie with other people who have seen it; it is as funny as almost any movie made. Borat is the same type of movie.
For whatever reason the actual film itself as a comedy is not great, but since they are essentially a long series of skits strung today to try and make a movie, the skits that are funny are even better when you talk about them. If someone you know hasn't seen them, you want them to see it so they get the joke also.
I'm sure there are many other such shared experiences (and comic books themselves are like that) but these two movies I think will go day as movies that are funnier to talk about then actually watch as a movie.