One of the best FPS games ever, it will have you addicted in minutes!
The story is great. It carrys on from Modern warfare and you'll find yourself gripped. There are four difficultys. Recruit, Regular, Hardened and Veteran. On regular the story may take about 5 hours, but on veteran it takes a bit longer.
The multiplayer is even better. There are loads of different game modes, from Team Deathmatch to Free for All to Capture the flag! Upon gaining xp you will level up, gaining access to different perks and weapons. The maximum level is 70, but there is a prestige mode where your level returns to 1 and you have to unlock everything again.
The only bad thing I can think of is that you may get fustrated, on the story or online. In some matches online you will be dying constantly (in the first few weeks anyway) but will have fun anyway.
Overall, Call of duty is a game not to be missed.