Save your Money and get Call of Duty 4 instead!
You don't get much of a Story at the beginning of the Game. Just a bad looking Cut scene with some blinking pictures, a female voice that tells you what to do, and your own very masculine wannabe-cool voice that responds with useless one-liner. These Cut scenes are covering a rough story and build a weak bridge between two levels.
The A.I seems like a mix of scripted running actions and suicide intention.
The Aiming is.....nothing you can influence.
The different Weapons have no real practical use. Most of the time you just run around with one weapon shooting everything that moves until your ammo runs out. The option to customise your Weapon is as useless as the different Weapons itself.
All together:
When you buy SOF: Payback, you get a really nice looking brainless shooter full of an overstate amount of gore and no Story. After a few levels you just loose the appetite of killing more without seeing any reasonable Story that explains your actions and give them a background.
My Advice:
You will have much more Fun and a better feeling by investing your precious money in a Game like COD4 or Crysis(if your Machine can handle it :D)