The more I hear about it, the less excited I get, unfortunately. I love the idea of four-player co-op, but the episodic nature of the game turns me off.
The fifth-player-as-villain is an interesting idea, but I think it might be better suited to a separate multiplayer mode rather than the main game experience.
In general I think this game could be fun in short bursts with a bunch of friends, but I don't see potential for greatness.