Competitive multiplayer wouldn't be something I'd be interested in, but the game is almost screaming out for a co-op mode. One person could play as Shepard and make all the decisions, while I could play as Garrus. I'd stop playing singleplayer altogether and I'd just stick to being Garrus 100% of the time.
Those are my favourite. We had great fun playing Army of Two so I got the sequel for his Santa stocking.