Put a few hours into now - currently on level 9 and have got the three wings unlocked at the base. Tried it out for about an hour last night with a m8 of mine too and it's much more enjoyable than going in alone on single player.
However. I have reservations. My biggest concern is that this will turn out to be a shooting hybrid of a spreadsheet - a game that simply increases in difficulty by adding zeros to the health, hit points and damage of everything in it. Already, I've noticed that mere enemy grunts aren't too far behind me in terms of level. And when I come across a 'mini boss' - he is either the same or slightly higher level than I am.
This creates horrible flashbacks for me of Borderlands 2 and worse - Dead Island. With the latter, you could walk into a building with level 4 zombies outside, you gain a level when inside, turn around and suddenly all the enemies are now level 5.
An enjoyable game that does make. You might as well universally make everything level 1 (including yourself) for the complete duration of the game. When a carefully aimed headshot took out a grunt with 300 hit points - the same enemy now has 3,000 and requires several same headshots to drop, simply because you've levelled up yourself.
I understand that as you get better, the opposition has to, or else it would be a walkover. But shake it up a bit - have random encounters where a level 2, 4, 10 and 18 enemy could all be circling my level 16 character. Why do I have to go up against statistical clones of myself throughout a game?
Hopefully things will change, otherwise I will get bored - very quickly.