With a cast of now extremely famous actors (in France), and outrageous humor, this movie is a huge classic in French cinema, one of these movies that is shown on French television at least once a year.
Americans or people of other nationality, might simply not "get" it though. Its typical French humor. I, as a Frenchman, find some American movies like Napoleon Dynamite or Adam Sandler films to be not funny at all (as would most Frenchmen, which is probably why they're not exported here at a large scale), although they are apparently successful in the US. It may be the same for this movie. I'd say its a movie made for a French audience, and very few non-French people would find it that good.
This is essentially a gross satire of the German occupation, a very delicate period in our history. A movie like this couldn't be made in our too politically correct times. The most hilarious and classic scenes are towards the end of the movie, with the hilarious Jacques Villeret playing "Hitler's brother". This is to me one of the funniest movies I've seen, and I would definitely recommend this to everyone, but non-French be warned; it is clearly Franco-French humor!