Got it yesterday, played with my wife and with some pubs.
It's pretty rad, definitely. I only got the base game - I try not to get DLC on principle. We'll see if I cave further on in a sale.
You can play locally, with keyboard and controllers. I have two controllers and a keyboard, and they all seem to work at the same time, so I think you can get 4 people playing on the same computer without any problems. You will all share the account of the person who owns it though - no levelling up your own character (even if you own the game). You can also play in any combination of local and online coop (my wife and I, on one computer, joined other pubs together, including other people on the same computer). You can't play "cross-platform" between PC and PS3/4, unfortunately. There's no versus mode, but that's OK because friendly fire is a way of life. Shoot everything, blow up everything, scream a lot, laugh a lot, die a lot.
The game is massively cooperative, in the sense that the state of the galaxy is persistent and everyone (PC and PS3/4) is contributing to the same progress against 3 different factions in a billion different groups of up to 4 people, each conquering planets. To conquer a planet, you do a couple of missions in a row (missions last anywhere from 5-20 minutes). Once you've conquered a planet, you add some points towards conquering the area the planet is in, and you get an unlock. Once the bar for a given zone fills up, you've conquered the entire zone and move on to the next one. Once you're done with all the zones, you get to attack the homeworld. Instead of getting unlocks, you get double XP. If you don't conquer the homeworld within 48 hours, the aliens push back a zone, as far as I can tell. Also, I think in the PC version any planet conquest counts for galactic progression, whereas just local coop didn't for the PS version (might be wrong about this).
So, three different enemy enemy factions, persistent galaxy that resets upon winning, a hilarious backstory that parodies US foreign policy pretty admirably, tight, frantic gameplay, unlocks, local&online coop in any combination, !!FUN!!.