For addictiveness and concept, this game would get 5 stars, for it's ability to frustrate the hell out of you, no stars!
Don't get me wrong, I love this game. Potentially endless hours of mindless gore and zombie mashing with as many different weapons as you can imagine. However, what ultimately lets this game down is the constant battle against the clock and poor save functions.
Death results in you have to restart from your last save point, no continuation. So from leaving the safe house to get to a mission on the far side of the map potentially could have seen you kill 100's of zombies using many different weapons, only to run out of suitable weapons and die within inches of reaching the mission start point, massively frustrating! The fact you can only save your game after completing a mission or finding a toilet is massively frustrating!!...
With the constant ticking clock and required mission elelments to the game (failure to do these results in the game ending immediately), leaves very little time and room to free roaming (think GTA).
Online element is fun, competing in various events against people earns cash for use in the main game. The Bike slicing event is top notch and great fun.
In summary, great fun, but could and should have been better!