Call of Duty 4 is simply one of the best FPS' ever made.
The story is quite emotional, with an incredible ending, i don't want to spoil anything so i wont give away to much. But I'm sure you will find it very entertaining.
After you are done with the campaign (which is not short if you play it on veteran) then COD4 has a incredible multiplayer mode, which lots of people are playing, you rank up by getting EXP points through 55 levels of rank and while you do this you w2ill unlock new weapons, and perks.
All the weapons feel powerful and accurate, and the gun models are great, the games graphics are superb, and the maps are designed excellently. With a good amount of buildings, cars, ETC.
This is a great game all around, the frame rate is solid, and never falters, the graphics and sound are great (including the voice work.) And the game has one of the best levels i have ever played called "Crew Expendable"
You owe it to yourself to pick this one up.