That's the real trade-off between single-player and co-op. The AI is mostly intelligent, but it lacks the accuracy of a flesh-and-blood partner. RE5 is significantly more streamlined than RE4, and that's because the Capcom team have built a world designed to encourage two people to participate. When the game starts, Chris and Sheva start out with the same pistol. After a level or two, the game starts to nudge you and your partner to specialize in certain weapons. For example, you might be great with handguns and sniper rifles, while your friend is handier with machine guns and magnums. The system creates an effective interdependency.
The inventory system also encourages teamwork. Gone is the upgradable attache case of RE4 and in its place is a system that's conservative in battle, but flexible between levels. Before each level (or as you're restarting at a checkpoint), you'll pick out suitable weapons, ammo, and health. You can only load up on a few items at once, but you can store anything else for future use. It forces you to choose wisely carefully plan out your attack.
RE5 also seems to be built with replay value in mind, since the first playthrough should last a little over half the time of RE4's. The game maintains a persistent memory of your progress, so each treasure and gun upgrade carries over to the next run. It's great for single-player, but its value for co-op varies by player -- whatever state you've left Sheva in is how another person will encounter her when they jump into your game. Nevertheless, the game's persistence is a valuable feature that encourages you to come back and keep playing.
The levels, from shanty towns to oil refineries to underground labs, are something to behold. Although each stage is more linear than in previous titles (the game co-opts Devil May Cry's level-by-level ranking system), there's plenty to explore, especially if you're playing with a friend. It's easier to scour and collect when you have a friend making his or her own choices than when AI is following you. The stages are largely variations on the same idea (walk down a corridor and shoot or fight a huge monster boss), but the style of each level is unique enough to keep the action fresh.
Much fuss has been made over RE5's backward-looking control scheme, but there are enough combinations of RE4- and Gears of War-style control options that there should be something to satisfy your tastes and needs. I suspect that those wanting the game to be more like Gears or Dead Space might be upset that you can't move and shoot at the same time. RE5's tension and suspense are rooted in the stress of its action-packed combat. In other words, other games provide terror in different ways. The nature of this game, and its controls, lend it intensity.
