Will Ferrell and John C. Reilly have to be considered one of the funniest on screen duos ever. They play so well off of each other and I would give anything to see blooper reels of these two cracking mid scene. The two of them playing big kids who still live at home was so picture perfect. The way that they would go back and forth between hating each other and then being best friends just like typical 10 year olds was hysterical. The only knock I'll make against this movie is that some of the funniest lines were delivered after huge laughs and you end up missing most of them. Even when the movie seems to go down that unavoidable dramatic downward spiral - they still manage to rebound and make you laugh your ass off.
I fully recommend everyone checking this out. Its not for kids at all though. There are some really awkward moments and at the risk of spoiling it for people - you need to be warned that you see Will Ferrel's testicles. Seriously. You've been warned.