Maybe not quite as good as Gran Turismo in some ways, but the feel of the game is really good, and the custom decals you can add to your cars are unlike anything in any other game.
Forza 3 beats Gran Turismo in some ways, loses in others, but is a good Xbox equivalent for GT if you want a driving sim.
It's structured similarly for ‘campaign’ mode, except just missing a few things GT has like licence tests.