I've played GT4 and while it doesn't have quite the poly count of Forza or this game, but the gameplay is quite impressive, esp. with the 900 degree Logitech Driving Force Pro wheel. Even though the poly count is lower, the dimensions and renderings of the vehicles and scenery look more realistic than the screenshits that keep getting posted of Forza.

My quick two cents: Good looking graphics from those shots, but it lacks the time-bred pedigree name of "GT," and for that reason alone it will not be serious competition for GT4 although it might rob a few sales from it.