First of all I should probably mention that I'm a fan of the whole zombie genre in general, which might give me the ability to endure and forgive more shortcomings than someone who isn't as much into the whole zombie stick as me might not be as willing to overlook.
It's good for what it is, though I'd personally wait for a sale before getting it. I have 40 hours clocked in the game - played since Early Access - and got it cheap while it was 75% off during the Steam winter sale (apparently they fucked up the discount percent back then :D).
There's 3 game modes with 1 being pure PvP and the 2 others are mixed PvE and PvP in the sense that as soon as you die as human, you respawn as zombie and from then on fight against the remaining human players along with your undead AI buddies. I personally don't care for the pure PvP mode so can't tell much about it.
The AI zombies themselves are not really hard to kill even on the highest difficulty setting, since 1 shot to the brain (it's actually the brain, not the head - so the hit box is relatively small but zombies are very slow and allow you to take your time aiming / letting them come closer to get a good shot off) is enough to put any zombie down. Their damage scales up and the amount of ammo available scales down. But not the number, HP or types of zombies. Because of the insufficient supply of ammo on higher difficulties, you'll have to resort to melee. Though here's the problem about the melee system: each melee weapon kills zombies in one single hit to the head and once you have the swing timing down, you pretty much will never get hit by AI zombies. At this point the only real danger comes from other newbie players fucking up and turning into player controlled zombies. Who's difficulty can be ranging from easy prey to deadly mofos, all depending on how well they know the maps (as in the good ambush spots) and how good their A/D dodge spamming skills are.
One of the PvE modes also gives you AI controlled human survivors with unlimited ammo. On top of that they are decent shots, making that particular mode an absolute cake walk.
The only worthy mode to play is called "Escape" and for this mode, there's an underwhelming number of 2 maps available. Even with parts of the maps being randomized you'll quickly see repeating patterns as there's 3 different main routes per map while the majority of a run-through takes you through the same areas across all 3 different routes just with different paths open. So instead of A -> B -> C -> D -> E, another route will be A -> C -> B -> D -> F
But there's light on the horizon, a third Escape map is being worked on and they have released the SDK which means the community can create maps of their own.
In regards to visual variety you got 8 different player models to choose from, each with their own voice actors (all doing a sup par job though sadly) and a decent amount of different Zombie skins. The maps are urban brown-greyish with a little splash of color here and there. A little disappointed here, not gonna lie.
There's also an alarming lack of variety in the zombie type department. There's slow zombies and very slow zombies. That's it. Once again though, the SDK has been released and I imagine someone will do something about it, just not the Developer :D
So hopefully we'll see something like fast zombie dogs, HP bloat body builder zombies, silent zombies or what-have-you soonish.
The community is small and the majority of players use the official Servers. This is a problem because for some reason, 98% of the official Servers run on the normal difficulty, so getting a game going on anything higher than the normal difficulty in the game mode you would like to play when you fire up the game can take quite some time.
It took me a good 30 minutes to get another player to join my own game called "HARD difficulty, don't be afraid <3"
When you create your own Server/Game you can also choose to disable player zombies, though that means dead players will run around as cockroaches on the map and in that case pretty much everybody that dies will leave the Server immediately because running around as cockroach in a game that might take another 20 minutes to finish is not fun. There's not even a free-roaming spectator cam so if you are unlucky you'll spawn as cockroach on the floor in an enclosed area trapped for potentially 20+ minutes without the ability to bypass doors (which, would you spawn as player Zombie could smash down).
All official Servers have Player Zombies activated from what I could tell. At least in my 40 hours - mostly spent on official Servers - I never played on a Server that had them disabled.
Another Server/Game option is to disable player infection. Once again, this seems to be enabled on all official Servers. The mechanic here is that with each zombie bite you receive, you also run the risk of getting infected by the zombie virus. Lower health means you have a higher chance of getting infected.
Once infected, you'll turn into a zombie after X seconds. There are medkits, but they only heal health, they do not cure the infection. So getting infected is an instant death sentence. In my opinion this is a very questionable design decision as medkits are hard to come by anyways (only 2 to 3 spawn per map for 8 players on a full Server).
All in all and despite all the improve-worthy stuff mentioned, I personally had fun with the game. Now that I played through all of the available maps and routes at least once though, I'm not feeling any urge to touch the game until the new official map gets released or some community created ones pop up. Then I'll fire up the game once more probably.
TL;DR: Modders will fix it; best wait for discount at which time more content hopefully will be available as well.