Bioware have chosen the safest option: carefully sectioned-off co-op parts that keep in touch with the canon whilst staying away from Shephard's most precious bits.
Introduced as the first part of Mass Effect 3's Galaxy at War system, the co-op on offer has an interesting twist. Max out your online warrior's level, and you can send them off into Shephard's battle - an invisible war asset that will make your chances of beating the Reapers in the single player story ever so slightly better. Bioware is quick to point out that it's entirely optional - anyone determined to get the best ending in the final chunk of the trilogy without faffing about with online nonsense can happily stop reading here and forget that any of this even happened.
For everyone else - let's get into the details. Four-player co-op is limited to wave-based skirmishes against the Reaper forces, but there's still plenty of scope for customisation. First up, you've got six of the series' best-loved races to choose from: Asari, Human, Krogan, Salarian, Turian, and Drell. On top of that, you've got XP to earn, skills to upgrade, and weapons to toy with and modify. That last part in particular is vital - as this mode revolves purely around combat, the inclusion of Call of Duty-style gun mods and tweaks seems like a bright idea for keeping things fresh.
Mass Effect: Galaxy at War
With eleven waves to fight through in each of the galaxy's many scenarios, things quickly get out of hand if you don't stick together. Difficulty isn't scaled when you're playing with fewer people - so we'd recommend getting a full squad together before stepping into the fray. Mixing up class-types is essential once things start to get hairy. In our hands-on session with the game we chose to play as an Asari Biotic user - keeping enemies at bay with powerful psychic attacks whilst our bruiser buddy hammered away with the concentrated fire of a heavy weapon.
Even with four people sticking together, things quickly get out of hand. Giant enemies charge into battle causing serious damage - and in the later stages of each round it's challenge enough just trying to revive the buddies who got left behind. With some waves demanding that the team activate specific consoles across the map, there's no chance of just bunkering down either. Keep together, and keep moving.
In terms of how it feels to play, there's little to be said that you won't want to hear - if you've ever imagined what it might be like to blast through Mass Effect with a couple of buddies, the end results are quietly pleasing. Full campaign co-op might not be on the cards - but frankly we reckon that's for the best. They'd only end up sleeping with your secretary and forgetting to feed your fish.
Mass Effect 3 will launch in March 2012. There'll be a multiplayer demo beforehand so we can see what's what.