Granted, there is a terrible balance issue when it comes to the assault class being able to go invincible for half a minute with no repercussion, the game is quite fun.  The fact that 2 teams are fighting each other while a third, npc group (the zombies) remains a threat to both sides makes everyone have to think about when they engage others.  Sure, zombies can be killed with only 2 button presses, but do you have the time to make that execution while others are gunning at you?  Not to mention you can even use this neutral third party to your advantage to slow down your opponent.  The single player was eh, but I’m thoroughly enjoying multiplayer.