I bought it earlier today and played through the first 2 levels.
It's got 4 characters to choose from, each with their own focus on a melee, ranged, or both. They each have their own special attack that builds up by killing things, and you can unlock weapon upgrades and perks just through killing as well. There's no xp or leveling though, you just unlock things by killing (maybe by score? not sure).
It comes across as a little hand-holdy though. Every time you get a new objective, you get a slow camera pan to show you exactly where to go, with a voiceover telling you what to do. It would have been nice to have those skippable. The important part is you play as a Space Marine, and you kill shit. I think it's got 5 or so levels, and the first 2 took me 20-25 minutes each (while searching for hidden stuff a little). So maybe an hour of gameplay if you only go through it once? If you do the whole thing on each character it would be 4 hours though. Except that I think you unlock stuff for all characters simultaneously. I'm pretty sure I unlocked some axe thing for the melee character that starts with a chainsword, so it's not like I'd need to play as the melee guy to unlock his stuff.
It's not amazing, but it's competent. Oh, and it's Warhammer 40k. I'm not experienced enough in the universe to tell how accurate this is, but it comes across as just as cool as I ever thought it was.
Oh and you unlock a powersword for WH40k: Space Marine by beating any level in Kill Team. It's not much, but it's something. (dear Space Marine, please kick ass)
EDIT: Yea coop seems to be local only. When going to the campaign it doesn't give any options for a second player until you're in the character select screen, where it says for player 2 to press A to join. In onslaught mode it won't even let me select coop without a second controller turned on.
