This has given me some hope. From what I read, this looks good, not bad. Looks like everyone is being doom and gloom for no reason..... http://techzwn.com/2011/10/mass-effect-3-dev-explains-how-mp-affects-games-ending-video/
link a guy writes about his hands on of the 4 player coop. says it doesn't detract from the sp and is more of akin to "its popcorn along side the
square meal of the sp campaign (assuming the sp is the same as the previous two in terms of quality)", slightly paraphrased.