Coming from a big fan of the Call of Duty franchise, it's a disappointment.
CoD4 came along, and was amazing, becoming one of the best and most revolutionary games ever made, with a dramatic single player campaign and a refreshingly new multiplayer.
MW2 came along, again with a great plot (albeit disappointingly short, I finished it in around 6 hours) and an even better multiplayer (in my opinion, at least).
With all this, I consider myself a fan of the Call of Duty franchise.
And then came MW2, and I'll tell you right now, I gave up on CoD multiplayer with Black Ops, so the only reason I spent 50 dollars on MW3 was the campaign. It was the worst 50 dollars I ever spent.
The campaign, while I loved finding out how Soap's and Price's saga culminated, and especially how they brought back characters from the original MW (Wallcroft, Griffen, Macmillan, and Kamarov), was depressingly short, clocking in at a record short 4 hours on my first playthrough (not the kind of record you want to break).
If you've played through the first two MW's and want to see how it ends, I recommend a rental, not buying it full-price.
As for Multiplayer, I hate it. The only gametype I actually enjoy is Infected, and the occasional private match.
It's doubtful you'll ever get into an actual firefight, as it only takes 2-3 bullets to kill you, and because of the natural lag that occurs when gaming online, they'll see you and be able to shoot you before you know they're running around the corner.