This is a draft review, I'll rewrite this word-hurricane later, treat it like a blueprint.
The game is late by about 8 years in terms of graphics, animations, gameplay and just about anything else you could name. It costs about $20, but for that you get about 3 coop maps and... 3-4 PvP maps. That's it! The game modes do not differ greately, except the coop ones of which there are two and they get boring rather quickly. Maps do change, but the routes are easily remembered, there are about three variations for each coop map, but if you ask me an occaisonally open door that was closed before does not make much of a difference, the tasks are varied but like I said only change twice or thrice per map.
But this isn't even the biggest problem. The worst flaw of the game is what is meant to be its selling point. The devs have made the multiplayer system so difficult and broken it is impossible to join your friends server (trust me I've spent 4 days trying every single solution on forums). The only way to do so is to download third-party programs like Hamachi and I have no intention to do that since I've had less problems joining in a 1999 System Shock 2. The devs are aware of this problem, but they don't want to fix it. Trust me, as I have spoken to them and they say they are denying any bit of constructive criticism. I think they are hinting at that "rent a server" feature instead. Pay-to-play? No thanks. And if you don't want a server and just want to play with your friends, be aware that numerous people will join as zombies 5 minutes before your escape in a 30 minute map and eat you and your friend alive, and they will pick your server out of about 500 empty ones available, yeah the community is nice too.
(A link to the discussion: http://steamcommunity.com/app/238430/discussions/7/540743212111472871/
This game is in development hell, essentialy you are buying about 5%-10% of what is promised. For this price, there has to be something like 50% more weapons, a campaign mode, 15-20+ maps, a working multiplayer, less bland characters that only differ slightly in voice and hand textures. But more than that, it should be somewhat playable because the balance is broken hopelessly (I'm talking about coop, since nobody buys these games for PvP), three guys vs. three zombies is an impossibility. In fact, one zombie player against 5 survivors gives the zombie about 70% chance of winning, as I have never seen zombies lose unless it's near the very end of the map. The maps are confusing, the directions are seriously broken, people spend 30 minutes trying to fight off zombies looking for an escape truck just to find it half-way through the map in a random detour.
"Realistic" zombie games were the talk of early 00's and many mods were made for a number of them, some were free and more complete than this. If it came out then as a mod, I'd download it and spend maybe two nights for fun, but to pay for something like that in 2014 - no thanks.
The fact is, the game is hopelessly overpriced and late and devs need a reality check. It has no point and no outstanding feature, you begin to wonder as to what the actual point of makiing it was. It isn't realistic enough to be the Arma of zombie games, it's not arcade enough to be for ♥♥♥♥♥ and gigles like l4d, it doesn't have much content to brand it as something inbetween these two zombie extremes. My verdict - the game 's like a random bored zombie that won't even chase you after you poke it, you can just walk past.
If you do not believe a single word I've said, look at the percentage of people that have got the easiest achievements for this game. It's in the decimals for most of them, because I'd bet this game is "enjoyed" mostly over LAN by devs and their friends, and anyone like myself who likes to buy random stuff and make reviews knowing even before the purchase what the game will probably be like anyway.
A little edit: there is a positive review up on the site, that is more popular. It was not removed, although it is early access, unlike reviews by Mantis and other people which were outright banned. Although I might see the logic that it was perhaps, not entirely about the game and more about the attitude of the development team, they were stating an opinion and so should not be banned. I don't see the devs removing the positive Early-Access reviews, so this is hardly fair.