Early Access games come in all manner of different states. I would say this one is in a perfect place to be entering Early Access. The game itself is already playable and fun, but I can still see where they can add even more in the future. The co-op works flawlessly, you don't have to jump through any hoops to play with a friend. I ran into very few bugs while playing and none of them were game breaking.
Over 9000 Zombies! is a Tower Defense like game, where the thing you're defending, in essence is yourself. You can place turrets, of which there are currently 6 to choose from, you can place a block barricade, and there are roughly 20 weapons to collect. Zombies drop health, grenades, scrap (with which to build), and on the rare occasion, new weapons.
I played only co-op, but I can see it being a little difficult for someone in single player. After one play through, we found ourselves searching for a more defensible building, as well as working out strategies to survive specific zombies on later, tougher days.
I look forward to future updates. I hope to see more turret types, more maps, a map editor, perhaps more zombie types and more zombie 'bosses'.
Overall, I highly recommend it!