I think I posted a journal entry about this not too long ago - we watched it with our downstairs neighbors who are huge fans of the movie. It was almost like watching it with commentary.
I don't think it's that funny if you don't already know Trey and Matt and their style of humor. But I really liked Orgazmo and I LOVED Baseketball, so it was right up my alley.
I'd really like to watch it with the commentary, I heard it just gets funnier and funnier.
Fun fact: The little jingle you hear at the end of South Park, when they show that image of the airplane that says "Brannif: Believe it!" - that jingle is from the main theme song from Cannibal.