While combat is fast and furious, with one or two bullets almost guaranteeing a kill, the AI is schizophrenic at best and can kill your members at a moment's notice without them having the slightest clue as to how they can react to an adverse situation. For example, suppose you had a team of four members moving down a corridor with rooms on either side. Let's say the team is moving to the other end of the corridor, and a terrorist is in one of the side rooms. As each member goes by the room, the terrorist guns him down. He doesn't react to the fire except by rushing toward the next waypoint like an eager lemming.
Grenades seem to have an explosion range of about two feet.
Obviously, lots can go wrong in a mission. You will spend lots of time refining your mission layout, which really gives the impression that the missions are puzzle-oriented, which, in a way, they are. At times, it can get very annoying and frustrating. Then again, if you treat the entire game as things would happen in real life, Rainbow Six would be insanely difficult - with all its faults, it's fairly hard enough already. Replayability is also helped by the fact that there are a lot of different ways to complete a mission, and they get much harder as the difficulty level goes up.