You pay for the multiplayer: the campaign is just free bonus content.
You play as Alex Mason, captured and put in an interrogation chair. As you are questioned you'll remember several episodes of your life in different missions and years. The campaign is well-written, despite the fact that the levels were built around their multiplayer counterparts. You do get memorable moments as well as one or two epic plot twists.
Gameplay is the same as MW2, WaW, CoD4, etc. You shoot, you crouch, you go to prone, you Aim Down the Sights and you also die.
Dying is the worst part of the game, especially in Veteran, where the stupid AI and the continuous enemy NPC respawn take a major role.
Multiplayer makes the player feel that the Campaign is just MP maps with a story chained between them. Innovation is the word. No more heartbeat sensors, no quick-scopes, no more nukes. It is CoD4 again, only better. You still have Chopper Gunner, but you have RC-XD. Create a private match with the RC-XD timer off and rewarded at 1 kill, then make a car festival. Fun guarenteed.
Oh, you also have CoD4-style barebones, you have Wager Matches and endless customisation. You will not regret buying, unless you don't have an Internet connection or the minimal skills to play, but you can always train with Combat Training.