Who doesn't want to play a game where there are thousands of zombies walking around, just waiting to be killed? The problem is, this isn't exactly what you get.
Unless you complete the game to unlock free roam mode, you are stuck with the very limited story mode. Missions are timed, therefore limiting freedom. Add to this an awful save system, which has 1 slot only, and save points which are out of the way, and you get a game that has a lot of potential, but has too many frustrations to make it appeal for long.
Also, unless you have an HDTV, you cannot read the text in the game to work out what the missions require. Not an issue for me, but I'm sure it is for many people.