Well for starters, zombie games are not really my type of game (with only very few exceptions) but I will state that after playing this for about 2 hours or so I finished up all 18 of the challenge/campaign style missions. The game is pretty much set up like so.
You spawn in a small boxed off area and zombie spawns attack, they will drop random power ups and weapons. Fend off all zombies and move onto next area.
Sounds boring but in short play bursts and the fact it is co-op = tons of fun when you need a quick fix of action. The controls are very solid, the weapons are very satisfying and the GFX are well done. The game itself is very inexpensive which makes it a good buy. I picked it up as a Steam special for $2.50.
some more fun stuff soon.....