Well, i just got the Xbox magazine demo disc and Forza is in it. The demo contains 1 track (Seca) and a bunch of cars. 350z, Scooby, Lancer, Corvette, 360 modena, 911, Viper '05, GT-one and others of its kind.
I haven't played the latest of GT4, being a NTSC dude, but from what i've seen of the home-made videos and the comments on handling and braking... it seems Forza is pretty much in the same vains. I'm used to Seca on GT3, which ain't really realisitic, so i pretty much huged sandpits for a while. You have to break a lot earlier, you can't take the corners at very high speed, especially the 6th corner which you technically can take flat-out in GT3. Camera movement in pitch and yaw is the same as in the GT4 videos.
In the good benefits for the game, its got Custom car mods (body and engine) and its got Porche and Ferrari licenses. Both which GT4 doesn't have (at my knowledge). Some of you will contest that the car modding is useless; well its better to have it then not, and its a plus for Forza.
But on the down side, i haven't seen GT4 on my TV, but based on the B-spec video Graphics, Forza is way behind in terms of details, textures and colours depth.
one funny thing in forza, when 2 cars hug, there's this WALL of sparks coming from between them. Its like someone's forging them together or somethin'.