For me, the basic mechanics of the gameplay in FIFA 12 are probably the best they've been in the FIFA series for some time. The passing and movement is a lot sharper, especially if you speed up the gameplay in the settings menu. It's about as close to old school PES as the Xbox has got in my opinion.
While playing friends is great, however, I really find playing the computer more of a chore than anything else. The main problem is the computer AI, which has been designed in such a way that it saps most of the enjoyment out of the game. It's not that it's too difficult, or too easy, it's simply that the way the computer plays is exceptionally tiresome to compete against.
They relentlessly try and keep the ball, but not in a realistic way like Barcelona or Spain would do - instead they run around in circles in such a manner that you can't tackle them. On several occasions I've chased an opposition player all the way back from the halfway line to his own goal line - as if simply running backwards is a legitimate strategy because it allows you to keep hold of the ball.
They also seem to have a perverse love of dribbling along the side of the pitch so that even if you do tackle them it spins out for a throw in. It's neither realistic nor enjoyable to play against this sort of thing because, especially on the more difficult modes, you'll find yourself giving the ball away and then chasing shadows for 2 minutes while the computer potters about without even attempting to threaten your own goal. It's not dominating possession so much as it is time wasting.
For that reason I've given it three stars because it's effectively written off the single player mode for me. As a multiplayer the game is fantastic, though, so if that's your main intention in buying it then I'd recommend it wholeheartedly.