Excellent game
Graphics, while not perfect, are very good. There are glitches, but these are very rare and are most probably caused by other things, not poor quality of the game, since I was unable to find them again. The game's graphics are backed up by the flawless sound effects and voice-acting. Being in the middle of a firefight with many opponents is likely to confuse you with all the gunfire coming from every direction, just like it would do in real life.
The game's single-player mode has a plausible story to accompany it and is entertaining while it lasts, which, unfortunately, isn't very long. You'll end up asking "Is that it?" when you finish the single-player mode. Certainly, the single-player isn't what makes COD4 buying.
It has a very strong multiplayer community, with well designed maps and different weapons to keep you playing. Playing against other players is much harder, much more exciting, and much more entertaining than the somewhat dumb enemy AI of the single-player mode.
If you're a fan of FPS games and are looking to play against other people for a long, long time, this is the game for you.