Well... Co-Op is (I think) freakin' awesome fun. Due to the similarities to Max Payne 2's DMW mode, I had huge gobs of nostalgia splatting all over my Joy Lobes (that is not a euperhmism). This may have affected my judgement somewhat... so, you have been warned.
I played it solo and with a little buddy. Essentially, it's all the single player mechanics (including last stand) in an enclosed area. Be prepared to put in some serious time because, if you play your cards right, you can last ages. You will feel like an utter badass.
You play as either Max or Passos, using any of their skins you unlocked/downloaded in the MP. You'll be attacked by waves of mooks, each getting progressively better armed and better trained. It gets pretty manic. What is quite nifty is that all of the mooks use the stuff we use with our avatars. After a few waves you get a Boss Round, culminating in a bunch of Bomb Suit wearing nightmares. Needless to say, they take quite a few bullets to take down.
As you kill more mooks you build up Grit which allows you to either unlock doors leading to other areas of the map or open crates that can have weapons, weapon upgrades, painkillers or ammo. This means levels have been tweaked and adapted specifically for this mode (extra doorways, floorboards, crates in walls etc), making them seem - oddly - larger. It'd be nice to have more than two levels but it's much of a muchness, really.
The presentation is quite snazzy: you have a helicopter following you about with a spotlight, offering news-style commentary on your spectacular kills. As you'll likely find out, the helicopter acts as the "kill" cam, which is a pretty cool touch. You see, rather than die in Co-Op, you get captured. The other player has to get to you and release you, which can be a bit of a mission in the later waves. The captured player watches all this from the helicopter camera. Like OJ Simpson without the car.
I haven't tried the other modes yet and frankly don't expect to anytime soon.... I think this one will be taking up most of my time for the foreseeable.
I'm in love all over again.... if only I could use my Mona in it.