I thought it was pretty funny, but its a very stoney movie. You're not going to get half of the jokes if you haven't delt with drug dealers or the stoney lifestyle. Some of the subtle things in the movie were the funniest, but there is no way someone would get it if they haven't spent a lot of quality time with a bong.
So, basically. If you smoke weed its worth a watch. If you don't, its a toss up.
I think Grandma's Boy is a much funnier stoner movie, imo, and seems to appeal more to non-stoners.