Mediocre, but remains impressive...
It was a fun little experience playing the game, the '3D' effect the playing character and zombies have is visually appealing for the most part, which is a definite plus to the experience of playing, you can tell that time and effort were actual put in to the flash through that, and it wasn't some quickly thrown together game.
The levelling system is okay, but it would be nice to have some customization as far as that goes, rather than having your gun level up on it's own - Though besides that, complaints are minimal as far as levelling up goes.
The game gets quite devastating once the difficulty boosts up a few times, especially if you are anything like me and refuse to enter the portal immediately after it is available. The player doesn't get swarmed by many zombies necessarily, but a fair amount that are difficult to take down can give quite the rush.
My main suggestion would be to add some customization to the levelling system in the game, and also the option of choosing your player's look, and avatar at the bottom of the screen.
Also, something you may want to look at, is the game does not fit the default screen size of the Newgrounds 'player' (if you can call it that), it is much smaller and this can be a pain. I'm not sure how to fix that, but if it is possible - It would make the experience a bit nicer.
Overall, quite fun, worth trying.