Ok so I've put some time into Forza 5 and I'm kinda mixed on Drivatars. They say that the upside is that it's better than straight AI because it provides a more dynamic experience. What I'm seeing in practice is that it makes for far dirtier race. Cars consistantly batter each other as if they are fighting to maintain a predestined race line in spite of the cars around them, and that line isn't always the optimal race line. It's just strange. It's definetely more dynamic, but I'm not sure if it isn't at the cost of a little authenticity. If real drivers raced like that fatalities would be through the roof.
That said, the player side is top notch. This game rewards clean driving and the vibrating triggers is a ridiculously good feature.. I wish they had that in F1 2013. My first couple races were horrible as I struggled to reign in my control aggression but once you start driving the car instead of trying to bend it to your will (i.e. Grid 2) it's like night and day.