If you get the feeling you've played this game before... you're not wrong.
And every time, we're wrong.
Modern Warfare 3 might as well be an add-on to Modern Warfare 2 because it feels and plays pretty much exactly the same. There've been no gameplay additions, the guns have changed but little (though there are a few nifty things like hybrid scopes). The multiplayer feels like Black Ops (I didn't play MW2 multiplayer), though the leveling system has been altered, probably for the better, which is probably the only positive change made--all others being neutral at best.
Unfortunately, this game is going to have to stand on its multiplayer almost exclusively because the single player elements are rather lackluster. Spec Ops mode is competent enough, though the scenarios lack a bit of the touch and flair that those in Modern Warfare 2 did. And the single player campaign is just a mess.
For a game that has tried to sell itself as having an engaging story (as indeed Call of Duty 4 did, and even Modern Warfare 2 to an extent) it falls far short of the mark. Every moment feels like someone's trying to impress you with an amazing visual or astonishing development, to the point where it frankly becomes dull. On top of that, they've wasted one of the best aspects of the first person perspective, immersion in the characters, by providing you with playable characters that you quite frankly have no reason to care about. It tries to create twists and turns in an effort to shock the player in the same way Call of Duty 4 did, but their attempts are either blandly predictable or just infuriating.
In the end, the entire game feels like it was put together by someone who tried to paste the best elements of Call of Duty 4 and Modern Warfare 2 together, but utterly failed to understand what made those elements work. A completely uninspired, lackluster installment. If you must have it, rent or wait for a price drop; it's just not worth your money.