Co-op Iron Brigade is a lot of fun!
GFWL is not dead. I had a working GFWL install from playing FUEL, and was up and running in 5 minutes. However, if you haven't, you probably need the updated client, which is here: http://www.xbox.com/Live/PC/DownloadClient
You can probably download and install this before you download iron Brigade, or you can install Iron Brigeade, uninstall GFWL, and then install the newer version. Make an account, copy and Ctrl-V paste the serial, done. (GFWL often takes a minute ro more to connect, and sometimes needs more than one attempt.)
You need to play two missions in singleplayer; after that, you know the basics of the game and get access to the "radio room" where you can invite friends, search for games etc. "Friends" means GFWL friends and not Steam friends, of course, so you need to add each other on GFWL. You can do in-game text-chat with "T"; if you have configured speech in the GFWL settings, you'll also automatically have voice chat with everyone in your game (if your microphone isn't claimed by Steam). The co-op missions are the same as the singleplayer missions, so you advance in singleplayer as you play co-op.
I like the co-op rules. You need to collect "scrap" (= drops from the enemies) to be able to build turrets; any scrap you collect (or that scrap collector turret collects) gets shared equally (as are the random chest drops), but any turrets you place go off your own budget. You can only build those turret types that you have equipped (they're called "emplacements" in the loadout screens), but you can upgrade any turrets on the battlefield regardless of type. Of course you can revive each other.
Tactics for us was to go into each new map with the same loadout as the previous one, slightly modified by the hints the game gives you. We were only defeated once, but then we do go back and refine our strategy for gold star performance. This means talking about weapon and turret loadaouts, turret placement strategy, fire sectors (very loose), and the like. We were already on the chassis that has a maximum of weapon slots and a minimum of turret slots, but careful planning was still necessary to get that coveted gold star: even at that extreme of the game balance spectrum, you can't just adopt "shoot everything that moves" as a viable tactic.
Game balance is excellent: chassis types allow you to trade weaponry (fps) against turrets (tower defense) -- this affects both the turret types you can equip and their cost. Special skills are equipped via legs, and they're balanced as well: "quick reload" means you're stationary while that skill is engaged; "sprint" means you're fast, but can't turn while running, etc. Yo need to use your skill wisely, i.e. tactically.
The "stutter bug": 6 hours into the game, after 7 missions, the game started stuttering when scrap is being collected. That bug is not present in the Xbox version, and the FAQ says that since it seems related to GFWL, it is difficult/impossible to debug, and Microsoft is not being helpful. So that bug is there to stay. Since you can't shoot while collecting scrap, you tend to only do that when no enemies are around, which means the bug is an annoyance, not a showstopper at this point.
I got Iron Brigade for free when I bought "Hack'n'Slash", possibly because I previously owned a lot of Double Fine games, including at least one bundle (Amnesia Fortnight). Getting it on sale seems definitely worthwhile even now, but if you have an Xbox, you're probably better off getting it there, because of the stutter bug.
As a game, Iron brigade is exceptionally well designed: it is easy to get into, has tactical depth, and allows for a wide variety of approaches to beating a level that are well balanced among each other, both in singleplayer and co-op.