Call of Duty 4 is a great game that defines the FPS genre.
I think the one thing that holds Call of Duty 4 back is its story. Other reviewers will tell you that it is very intricate and has many surprising plot twists, but aside from a few witty remarks (namely by Cpt. Macmillian in the flashback level) I found the plot to be standard B- rated action flick material. I'm not saying that the story was bad, it just feels used.
In conclusion, this is a terrific game that lives up to any hype you may have heard (and there's a TON of it, especially since the game is over a year old now). If you have a PC that can handle it, then you want it for that platform because it obviously looks the best with high resolution AA graphics, but the console versions are no pushovers. My PC isn't top grade, as I have midrange 8 series graphics, but the game runs on high settings at a solid 50 fps with AA on. Buy this game!