I found the movie off-putting for a while, but eventually got into the movie, mainly due to the consistency of the crassness and vulgarity of the main character. Adding the slow overweight boy who (apparently) believe Billy Bob was the real Santa was enough to make the movie very enjoyable. I can see why people didn't like it, it certainly wasn't what I expected, but the writing style was very fresh and the movie moved along at a brisk pace, to the point where I was actually rooting for Billy Bob at the end.
It's the proper frame of mind. Note I said it was High Body Count Night, so the guys *wanted*, nay, demanded, vulgarity and crassness, and it paid off in a big way. Properly attuned, it's pretty hilarious. If you come at it expecting high wit or verbal hijinks, you will be disappointed. But I can't imagine watching it with a disapproving female; that would be painful.