The single player story and gameplay in this game is the best in the series in my opinion.
I played through the campaign twice and loved every second of it. But for me this game really came into it's own when doing the co-op challenges. For me this was more fun than playing the versus MP. Getting together with some friends and going head to head with increasingly smarter and tougher AI to complete all the challenges was some of the best fun I've had in any of the games I've played on Xbox Live.