i'm a bit disappointed with GT5 to be honest, after my xbox 360 decided to die (red ring of death) i got a PS3 for my birthday from the missus and got it with GT5, currently 45% through the game.
Got to admit that i rate Forza 3 on the xbox better than GT5.
What lets GT5 down is the modifications, can't do anything to brakes, engine conversions would be good, on forza you can fit different engines into to cars.
Weather changes and graphics are parts that GT5 does well and are better than Forza.
I'll certainly be buying another xbox in the near future as Forza 4 will be coming out soon!!!!