With an outstanding campaign and outstanding gameplay, COD 4 won't dissapoint you.
The game takes a radical departure becuase for once in COD histroy, it doesn't take place in WWII like I mentioned. Anyways, there is a civil war taking place in Russia. Somebody wants power, and wants to literaily kill for it. You play as S.A.S commandos or the U.S Marines. Either way, even during training, the weapons are awesome to use and awesome to fire, and to take the enmy down with. The story is well thought out, and has clever twists mixed in. Hell, in the first mission, when you're escaping, you'll have to make a "leap of faith". Don't worry, its all good. You'll love the weapons and the gunfights mixed in. It all wraps up with such greatness, you'll be like "I'm I even playing a Call of Duty game?" Seriously, its just that good.Though the campaign could've been just a little longer. You'll love the ending. I gurantee it.
In the end, COD 4 is the game that follows up Halo 3. If you've finished playing Halo 3 and have some money to spare, go ahead and get this awesome piece of work. With an outstanding story and outstanding gameplay, COD 4 won't dissapoint you.