Zombie movies are like cockroaches. They are ugly, hard to kill and if you've finished off one of them, two others are already waiting. They are everywhere and they all look alike.
Thus it's really refreshing to finally find a movie that successfully ridicules and mocks the genre with all its standards and stereotypes.
But the really impressing thing about this is, that it can do so without the least bit of irony. You will find nothing of the nowadays standardized sarcasm and sardonism in it. This movie is one of this century's last irony-free refuge zones. Yet it's even better than most sarcastic films of that genre.
I just laughed my ass off when I first saw this movie - and it's even better the second time, because you will notice a lot of funny details, especially in the beginning of the film.
The movie also greatly benefits from Sean Penn's amazing ability to look totally clueless and surprised. It's just gorgeous to see him wander about the first half of the film, not knowing what's going on.
I'd say: This is the best zombie movie in all of the 21th century (so far).