Red Faction: Armageddon is skipping versus multiplayer this time around and going with co-op instead. The horde-like mode called Infestation will see up to four players fighting wave after wave of progressively harder aliens until everyone dies or they pass the 30th wave.
There are three different game types; one that sees you killing everything, another that sees you defending bases while killing everything and then a dark mode where there is no light source except for each player's flashlight. Weapons are unlocked as you progress through the waves and all the upgrades you earned in single-player are available in the co-op experience.
If you're a fan of the horde mode in games like Gears of War and Halo, then you'll enjoy Infestation. The only downside to the mode is that there's no scoring or rating system, so you can't really get that competitive in the experience. It's a fun diversion but I don't think it's enough to satisfy the competitive players out there, especially with the loss of versus.
Volition wanted to focus on the story the most this time around and they felt that this co-op mode would be the best complement to single-player. As Roje Smith, Associate Producer on Armageddon, told me though, Volition will factor in a proper versus multiplayer if there's enough fan feedback in the next game. That is if another Red Faction game is made, of course