Ted Price is here in Sydney today talking Resistance 2, and he's added some detail to the nature of the dedicated co-op campaign mode. If you hadn't already heard, co-op is for up to eight players, but is a completely separate campaign to single player. That said, Ted suggests the co-op campaign itself will be good for around 100 hours of play, so this is no weak sauce mission pack (even if that is quite an exaggeration).
Co-op tunes itself to the number of players in the campaign, adjusting enemy levels to suit the team you have put together. There are also three classes in co-op: Soldier, Medic, and Spec Ops. You will need a mix of these, as Medics are your heal source and Spec Ops hold the ammo. The Soldier can provide moving cover. It was suggested that only running a two player co-op will be tough as you will be missing a class, but it won't be impossible.
More on replay value enhancements and experience rankings after the jump.Ted also discussed the replay value on offer in co-op, with branching storylines offering different experiences. Another big factor is random pathing through levels, with enemies spawned in different ways so you won't know where and when to hit them on a second run.
Experience rankings are maintained across all forms of the game. So what you earn in single player, you keep when you first take yourself online. Same goes across co-op. So you don't have to be an online fiend to keep up with the joneses, and you won't be hit with a nasty experience curve when you try to join in with all those other rabid MPers.