The feel of the weapons in Call of Duty 4 are extremely tight, but the AI was average at best. Even on the hardest difficulty settings they would still get out of cover mid-reload and fail to throw grenades back, not to mention long range shots with shotguns. Also the only real strategy the AI had was getting to cover as fast as possible or run into the line of fire with rocket launchers, not very intelligent.
That said, the single player really had some of the best moments in my entire gaming career and I play it to this day.
I'm really excited about Modern Warfare 2, can't wait.