I'm in the beta.
8p co-op is fantastic.
It's not like your usual co-op mode, instead implementing various class-based PvP elements to it. Only, instead of fighting another team, you're fighting massive legions of AI.
IIRC, there are 6 maps, based on the PvP maps. They're semi-randomized. Each map has a bunch of zones, and every mission you're being sent to different zones, so this should have some good replayability.
There are 3 classes.
-Soldier - big gun, and a secondary fire summons a shield which allies can hide behind.
-Medic - attack beam to absorb enemy hp and refill your secondary fire ammo. Secondary fire is a wide area blast which heals allies but not you. It's a good balance between having to shoot enemies to replenish your secondary fire, and trying to keep your time alive (you can also revive dead allies by going next to them and pressing a button, though anyone can do this).
-SpecialOps - Don't know the details, but this is your ammo guy. Keeps allied guns running, and replenishes shields as well, I think
It's a good balance. You need your soldiers to actually deal good damage and to absorb shots with the shield, your medics to keep people alive, and your SpecOps to keep the war machine going, pushing forward slowly on and on, as you're advancing your shield. Deploying shield near corpses so someone can get near to revive, fending off attacks from multiple flanks. You really get overwhelmed. Didn't get the party wiped out yet, but there's been some instances where more than half the team was dead. Good fun.
(re: Nick, medic feels a lot like the monk in GW in terms of awesomeness/keeping the team alive - only, without GW crap)