Resident Evil 5's story mode is nice, but versus is where it's at! The $5 add-on for Resident Evil 5 is awesome, and I've been playing it tons. It has a million problems, sure, but somehow it manages to be extremely fun.
Let me sort out some of the confusion of what we're even talking about here before going into the details. Resident Evil 5's main mode is a story mode that you can play single player (with an AI partner) or co-op (with a human partner) over system link, split screen, or online. When you beat the game, you open up a new mode called "mercenaries." Mercenaries mode is distilled to be just actiony goodness. No story, no RPG-leveling-your-weapons bullshit, no grinding of any kind. Just pure action and the challenge to score as many points as you can in a given time.
Remember, that mode is part of Resident Evil 5 (without buying an addon) and comes in single player and co-op flavors. There are 8 levels and 10 characters to choose from. Buying the versus mode addon allows you to play mercenaries mode with four players instead of just two and adds competitive options rather than just cooperative. You can choose between 2v2 and free-for-all (1v1v1v1). In either of those modes, you can choose between the horribly named "Slayer" and "Survivor" modes. My friends constantly forget which is which between those two modes, so we stopped using the official names entirely. My handy mnemonic is "Slayer mode is the mode where you don't slay the other players as much." We call it deathmatch (aka survivor) and either score attack or just "mercs mode" (slayer) because mercs mode used to mean the score attack mode before the versus update.
Free-for-all games inherently have the problem of being about who gangs up on who with alliances and kingmakers. I've played 2v2 about 99% of the time to avoid that, most of it with an awesome guy named Blicen who's in the military in real life. Maybe that's what makes him so good with the sniper gun, I don't know. We played the deathmatch mode a bit, and it's actually better than we expected, but score attack is where it's at.
First, a description of deathmatch mode though. You start with only the most basic weapons and there are three ways to get more powerful weapons during the match: a) kill a boss, b) pick them up around the level (not many of them though) or c) players drop them when they die. If you have at least one non-basic weapon in deathmatch, and you die, you will drop exactly one of those good weapons. There are zombies and bosses, but not as many as in score attack mode and they all have fewer hit points. Also, they have a 100% chance to drop ammo / items when killed, unlike the other mode. The point of deathmatch is to score more points than the opponent, and points are only awarded for dealing DAMAGE to enemy players (not just kills). This was a very good move.
Straight up deathmatch between players only would have been too shallow of a game, so the bit about zombies in the map with you and bosses you need to harvest for weapons while avoiding the other players killing you add what's needed. I don't think there's anything actually wrong with this mode, but a couple things make me not like it as much. First, because you start with only the basic weapons (handgun or machine gun), the character diversity is practically gone. Second and even more importantly, there's a lot of "cat and mouse" play. This is standard in many types of games, so I'm not saying that's bad, but the lulls of 30 seconds where one side is hiding/avoiding combat because they have a lead is just nothing like the excitement of score attack mode for me.
Score attack mode is basically just like the mercs mode that comes with Resident Evil 5, except you try to score more points than the other team. This mode is action packed and frantic. You must keep your combo going to score well (oh, I forgot to mention that deathmatch mode removes all concept of combos). Whenever you kill an enemy (either a zombie or a player), you have 10 seconds for either you or your teammate to kill another enemy to keep the combo going. Each kill in a combo gives you bonus points in addition to the regular points you'd get for killing that enemy outside of a combo. Anyway, you must keep your combo going while dealing with the enemy players trying to kill you (disrupting your combo) while you also try to kill them (disrupting their combo).
There is certainly player vs. player interaction here because of the points you get for killing players. The point values are tuned just right on players because they are worth just enough that killing players is worthwhile and good, but the points they give are also low enough that you cannot get by on player kills alone. You have to get a combo going with lots of monster kills. Every moment you aren't killing something is lost points, so that's why the mode is so frantic.
I know the game has major problems. The matchmaking is horribly implemented. You can't even join as a team in a team game. The ranked mode is a complete joke, allowing you to see everyone's name and kick them if you don't like them. It also doesn't allow joining as a team in a *team ranked* game. The ranking points make no sense to me at all, but who cares because ranked mode is jacked up anyway (player match it is, I guess). The inventory system in Resident Evil 5 is horribly clunky and really detracts from the experience. (Stuck in clunky inventory menu while people are shooting you is terrible). The balance is a bit off with Wesker being too good once you know a couple tricks, though I actually tend not to play as him much (more fun to beat him?). He has tiger knee and tiger uppercut (it actually says "tiger uppercut" on screen) so I wonder if he's a bit overpowered on purpose. And some of the maps favor certain characters waaaay too much (looking at you, Prison map).
I'm aware of all those problems and more. But it's still a very fun mode!