Pretty good
There's only a couple of things I would change.
1. Chat box for multiplayer!!! It's so stressful to watch people do dumb stuff and not be able to tell them to help them and keep the team from getting killed.
2. Please, Please! PLEASE! NERF THE SPAWNERS! It's one thing to have a mode where it's an endless storm of zombies and you are going to die no matter what, let's just see how long you can last. It's another when you say, 'Hey! Go kill those things to win!' and if you don't kill them in the first minute or two (which is nearly impossible because of how much health they have) and then to top it off, if you don't kill them soon enough they start spawning so fast you can't do anything about them because you're busy running from zombies.
Now, you either need to nerf them, or only send one at us at a time. We can deal with one, problems come in at two, and out of the nearly two dozen spawner games I've played, I've not won a single one that didn't have the help of someone who bought the 'Premium' weapons.
Other then those two things the game is awesome. Keep up the good work.