Picked this up, as a break from Division 2 - as that thing is getting stale!
Happy to say, it's actually quite enjoyable - reminiscent of L4D, though it could do with more than 12 'maps' imo.
The progression system is ok, though I can tell it'll start to feel quite a grind, not sure how many times you can repeat the same old maps to unlock the next level of a gun/character. You certainly need 4 humans though, as me and a mate got easily overwhelmed on Normal
Oddly, the AI players seem to do huge amounts of damage - handy if only a couple of you are playing I suppose.