OK..
This game's alright, graphics, concept, weapons-- are all good. I got all the weapons, and then eventually got bored and quit. I really didn't find this game to be very challenging, in fact it would be very difficult to die, I even left during the middle of a zombie attack to get some apple juice, then came back like 2 minutes later and still easily killed off all the zombies-- in like level 15 LOL...
An example of a challenging action game would be Tom Fulp's Alien Hominid, fast paced action on the edge of a knife. You need skill, speed, and keeping your wits about you to survive-- that's what makes a great action game. This game doesn't really have that, it's way too easy-- and that will eventually put you to sleep.