There are about 5 co-op missions, and about that many versus missions too. There is also good ole freestyle mode, in which you just wander around the entire city causing mayhem. Kor and I had a blast doing this, we traded places driving and riding shotgun. Literally shotgun (actually uzi or pistols). We also crashed a couple helicopters and I ran over Kor in a boat.
Everything is customizable in the multiplayer mode, too, so we could all just team up and take on the po-po.