So far the funnest purchase of 2010 (Minor Spoilers)
Basically, at anytime you'd like you can restart the game and use your characters money, level, items like car key's, etc. I usually hate achievements, but it's really fun to get them in this game. Over the course of the game you also unlock clothing, which can help make your character be the biggest "zomedic" clown, or the most serious zombie slayer on the planet.
The online is a very well-thought idea. The whole thing is competition between you and 3 other players, but you never fight them, it's all fun minigames, like in one you are in a hamster wheel and your bouncing off poles like in pin-ball to get points, or in another you are in a zamboni, you must run over zombies to gain blood and then shoot the blood at these statues with open mouths. And you know what makes it even more fun? The points you make get turned into money which can be imported into any save file you'd like. Genius.
The main story cases of the game have both great story and great challenge. I won't reveal anything, but be prepared to be pulled into an extravaganzic experience.
Overall, a super fun game and so far very worth my purchase. I have some very minor concerns with the game, but the glitches and game mechanics won't interrupt your playing time. Zombie mayhem has never been better before.