Konami's PES 2010 has some questionable improvements in the game's AI.
In the 2010 version, however, many changes were made in order to make you think and act more like a manager and less like a soccer player. For instance, if you put a player outside it's primary function, he'll have a penalty in his abilities, which is cool, if you look at it as it was a real soccer simulation. Not every player can play in every position of the field.
Another thing that affects players' performance is their morale. Until 2009 version, you didn't notice the difference of having a low morale player to a mid-low morale one. In PES 2010, it changes the players performance in a considerable way.
Something that also has a spot on is the players' AI. On the bright side, there's an effort of your teammates opening up spaces and running behind the defence to receive passes while you're holding the ball with some playmaker midfielder. Not that movements like those weren't in the other versions, but in this one you can see it more clearly.
Another nice thing is that only players that can really dribble or shoot do it right. You can't dribble with AC Milan's Gennaro Gattuso like you do with Barcelona FC's Lionel Messi, for instance, and that's pretty "real" if you ask me.
However, the good things stop there.
First of all, the player's AI. Some of the players get somewhat retarded when you're defending or surrounded. I can't see - in real life - professional players, in important matches like a UEFA Champions League playoff match, let the opponent pass the ball free of any pressure and with little or no marking at all. The greatest flaw is in Top Player difficulty: your teammates all contract some kind of mental illness, because they totally dismiss any marking or extra efforts to get the ball. And on top of that, your opponent's pass accuracy is almost 100%, which makes any opponent, no matter how weak, play like today's Barcelona FC, passing the ball quickly and leaving the other team dizzy in the field. You goal keeper is just a guy in the box that can put the hands on the ball. It's not a goal keeper per se, it's just a guy there.
In another flaw, there's some weird goals that you don't see in real soccer as often as you see in PES 2010. Like when you shoot and the goal keeper defends from behind the goal line, or your defenders all gather up and decide to make an own goal.
Sometimes you put a player running in the direction of the ball (with the shoulders' buttons, to move freely), in order to intercept a pass, and he just stops, maybe scared of the ball. That's just stupid. No player - even amateur - would ever do that.
Another one is when you're in Become a Legend mode. This mode is where you can notice most of the game's AI flaws, and I'm not going to describe all of them, but be sure you're going to get pissed - or laugh - a lot when you're playing this game.
When you got something really good and tries to make it better and better, that's when you go wrong and screw up everything you made so far. That's what happened with PES, and the 2010 version is very disappointing, a failure that is expected to be fixed in the 2011 version. Or so I hope.