Best sim available on XBOX.
It has everything GT4 doesn't..well.. almost everything. It has better AI, damage, more creative level design, more american, european cars.
oh, and tyremarks, too.
however, there are some things which could be improved. for example the phsyics aren't that good. Of course, it's realistic and all, but for example, the brakes are too weak. Almost every car feels a bit overweight, and slow in turns. The 30fps isn't healthy for the speed-feel also. The graphics are fine, but the cars should be polished a little more.
But don't worry. Forza has it all. Forza will keep any car freak entertained for long weeks.