"The first fear in relation to the co-op experience, that your partner Sheva's AI will hamper your effectiveness, is largely unfounded. Instead she is as useful as you want her to be. If you give her some decent weapons and take advantage of the basic commands system, she becomes very handy in a fight, particularly in regards to assisting you when attacked. The only time she gets unruly is if you walk too far away from her, which triggers her instinct to go collect items on her own, though this can be minimized by sticking together (which then allows you to choose which items she picks up).
No, the real problem with co-op appears before Sheva can even pull the trigger. Resident Evil has always struggled with item management, and RE5's new solution makes RE4's Tetris-like attache cases look heavenly. This time, all items take up one slot in your 9×9 inventory, but space fills up sporadically. You may find yourself with no room to pick up an important item, only to pass it on to Sheva for safekeeping. After an intense fight, you may find some extra space available, forcing you to swap items back and forth with her, and hoping she didn't chew up any ammo you passed on."