I tried this out for a while at a game shop. The DS@ was very hard to use with the game, but it was actually pretty fun. I expected it to be horrible.
As for the AI and difficulty, I just jumped into an unknown track for about 10 laps (imola), I wasnt fast enough to see how the game was when you are running up front with the other cars. I bet this is where the game dissapoints. GP4 has the advantage here.
As for physics, graphics and everything else, they were pretty good from what I remember.