I've been playing slowly since I got it, just hit level 50 a few minutes ago. I'm currently on race 20/24 of the Championship Series (year 9). I got my BMW E92 M3 at A600 and it is tuned absolutely perfect. I'm constantly posting times in the top 2% of any race and the car just feels "right". There are even a few tracks where I have the fastest BMW in the list. The only "assist" I'm running with is the clutch, because I can't easily do it on a controller. No traction control, no stability control, no ABS, simulation steering, etc. I love it.
I dislike the AI roughness, compared to Forza 3. If you are where they want to be on the track, you are getting hit. I'd say I'm restarting the race after the first few corners ~40% of the time because I either end up backwards on the track, lose a huge amount of ground from being pushed or rear-end another car because they slam on the brakes. Once I get in front, I'm fine.