The overall atmosphere made me think of a Stephen King-style wasteland (think Cell) and was really well done. It's not really scary or anything, but it still manages to capture the quick destruction of society and the wasteland/loneliness left behind.
And then, it's also hilarious. Ridiculously funny. All parts are flawlessly cast. Jesse Eisenberg is twitchy and neurotic to a new level, he just seems like his heart rate is constantly much faster than the average human being. Woody Harrelson is the perfect foil to this, constantly smooth, slow, confident, deliberate. All backstories are slim, but effective. The characters are believable, and it's easy to care about what happens to them, and they all seem to grow in some way by the end of it.
Overall, comparisons to Shaun of the Dead are obviously going to happen, since they're both zombie-comedies. I'd say this is the perfect American counterpart to SotD. Where SotD captures the conservative, restrained, polite British society, still worried about what others think in the middle of a zombie apocalypse, using whatever weapons they can find, Zombieland captures the big-truck-driving, heavily-armed, hot-blooded Americans, as well as the neurotic, shut-in-with-technology type with the paranoia handed down from his parents. Where SotD's comedy is subtle, Zombieland is more in your face, though it still has some brilliant subtlety and timing.