Ok, I know it seems like I'm really trying to get poeple to play this, because I am.
There are so many damn game modes for online, I can't keep up. It seems everytime I get invited or my posse leader switches, it's a different game. Now, that being said, most revolve around two styles:
Your posse against the CPU or Posse v. Posse. Granted it is 3rd person, but still fun. One thing I like is swithing game modes is about as simple as can be. You can go seamlessly from game to game.
And I'm not whipping a dead horse BTW, most of the time there are 8-10 poeple in each server. Max is 16; and 8 can be in the same posse.