Call of Duty 4 is extremely nice, it's similar to the Battlefield series except it's a little bit faster paced/sporadic (due to sometimes strange spawn patterns). You achieve ranks and are assigned different challenges (you can even unlock golden weapons if you're good enough). So yeah, gameplay-wise it's a decent game, but the "leveling-up" makes it addicting.
If you are a FPS fan, get call of duty 4 at least you will get the first prestige and do the single player game, which is around 40 hours or 50 hours long. Some people complete 10 prestiges in call of duty 4 and Ive seen people with up to 40 days of online gameplay.
the game is 40 dollars right now and you get the map pack with it if you buy the game of the year edition, its a robust game worth looking at if you want a good online experience.