Yeah, its pretty good.
Graphics wise, COD4 is just spectacularly stunning! I've only played the PS3 version, but I can assure you, you cannot be disappointed by how well the world in COD4 looks; HDTV or not, it looks good. Combined with a silky smooth frame rate, the COD4 world just looks very realistic and touchable. The textures are very high Res, especially on the characters and guns. You just can't not stop at some point in the game and admire the realistic look of the inhabitants of the world in the game. There are times you'll notice a couple bad textures here and there, but really, who cares? The good outweighs the bad.
There isn't much I can say ,story wise, about the single player in COD4. Its very well done. Although short (around 5 hours depending on the difficulty and how well you play FPS's) the quality of the campaign is very high. If you've played any other COD game before then you'd automatically know how COD4 works. You basically just jump into a stage and clear out all your targets within the first area, move on, clear out another area, and so on, until you reach the end of the stage. There are four different types of difficulty in COD4. There is the easiest difficulty (which I haven't tried out yet), regular difficulty, hardened difficulty, and veteran difficulty. The easiest and regular difficulty aren't much of a challenge, and are suited for those who haven't played an FPS before, or just want to go ahead and progress through the story without having to worry to much about the enemy AI. However, hardened and veteran difficulties shouldn't be taken lightly. Just by my experience through the hardened difficulty I could feel the large bounce in AI intelligence from the regular difficulty. Its challenging but very satisfying in my opinion and gives the game lots of replayability.
COD4's multiplayer is another of the products highlights, and is probably the biggest reason why COD4 has such a large amount of replayability. You basically start out at level zero and work your way up the ladder gaining XP from every kill you get in a game. Eventually all that XP you earn will reward you with a level gain, which will represent how good you are at the multiplayer. You can unlock different weapon customization options and perks as you keep playing online, which you can then strategically turn on what you want to make you have an advantage over the other players.
Overall, I believe COD4 is a solid game, with a LOT of replayability with its singleplayer and multiplayer components.