Well, I've been playing this game on and off for a few weeks now and thought I'd make some observations (mainly compared to GT5p obviously).
1. It's an arcade game - not any kind of sim. Though a lot of people say GT5p is not as "sim like" as some available on the PC, this is nowhere near. You still have to brake for corners, and take a half decent line but other than that it's not realistic in the slightest.
2. The damage modelling is outstanding - especially considering all of the major names in the game. It adds a sense of "fear" you don't really get in GT5p knowing that if you **** up (and choose not to use flashbacks) your face is finished.
3. This game was clearly not made with a wheel user in mind. It's very, very difficult to get a good set-up (I had to look online for some time before I could) and when you do, it's still hard to play. That said, it is significantly faster with the wheel, it almost seems like you can get more lock on the steering wheel and turn faster.
4. The AI is far superior to GT5p, but is still lacking the will to win. They put up a really good fight, until you pass them, then they give up.
5. Turning the aids off seems to slow you down, a lot. The complete opposite of GT5p.
In summary, it is a very good game. Well worth the £20 or so it's going for on some UK sites now. Don't expect GT5p style realism though, take this for what it is, an arcade racer with amazing cars and some really nice tracks (Spa, Donnington and the american street circuits are a right laugh). This is a vast improvement on the almost unplayable Colin Mcrae: DiRT - which was an insult to an amazing talent.