This game is considered a 'horde mode' game which includes games like Left 4 Dead. It's very co-op oriented and tons of fun in groups. Pretty good review from TB here, AngryJoe also reviewed it.
I actually have many hundreds of hours in this game, and am extremely good with the elf. On that note this game is literally left 4 dead but with scaven instead of zombies. It also unlike left 4 dead is heavy with melee, as the hordes that come at you at higher difficulty you will not be able to kill from range so you wean them then finish with melee. The special mobs are copies of the left 4 dead ones, from tank, spitter, etc. It adds an element of RPG as you can get better tiered weapons when you win a map via a random rolling event or via crafting in the lobby. I personally enjoy it, and i am sure they are doing this free to play because warhammer deathwing is coming out next month. Which looks similar to this game but more with bullets i think.
I bought it so I'd have something to play in between bouts of Division. I'll definitely play it with anyone after I stop this Division binge though.