>>363737273
I'm 13 hours in and still not bored of it. It has tragically few levels but they are kept interesting by the AI director and the desire to keep grinding for weapon upgrades. I love using the guns and the zombies with the gore system and all the climbing and crawling they do are fun to shoot. 

The questlet infestation and lobby system is bad though. I haven't gotten a single PVP game in yet as the host is always AFK and there's no lobby voice chat to do a basic sanity check. And of course higher difficulties are unplayable as questlets will join with no upgrades and generally play like subhumans.