It is quite fun. It is also quite gross at parts, but I think that this thing is a great deal of fun. It also shows how good ideas are kind of timely. A year ago, I was challenged to write a novel about Zombies. I realized that at the time there was a large void in the Zombie genre for protagonists that weren't morons. That was why I wrote Grenademan Vs. The Zombies. Before Zombieland, the main movies that did this were Army of Darkness (a classic but nearly 20 years old) and Shawn of the Dead (which was more of a parody.)
It is true that, like Shawn of the Dead, Zombieland has a bit of a comedic element, but despite that, Zombieland retains more of the true roots of the genre in my opinion. At no point do we really consider the scenario that they're in a joke. It certainly has humorous elements, particularly when they decide to spend some time at Bill Murray's mansion.
The heroes are human and flawed. And despite that, they don't behave like morons, which I greatly appreciate. They give an explanation about where the zombies came from, but it is shoved far into the background of the story where, quite frankly, it belongs. This isn't a story about the zombies, but instead the way it changes the people involved. It is a story of coming to terms with loneliness and the value that building societies can bring.
I greatly enjoyed this movie and highly recommend it. Not for small children though.