I've always been a Gran Turismo fan, and this is the first game that comes close to toppling that.
The graphics are fantastic, the interior modelling of the car is really impressive (they modelled the dashboard of each car). So impressive that I actually played the entire game with the in-car view.
The driving mechanics feel great... I played the game with all aids off except ABS and kept the braking lines on. I love having the braking lines. One frustration I always had with racing games was learning all the braking spots. I never had the patience to learn the braking spots for each car on each race track.
Also, having the ability to "rewind" whenever you mess-up is great. I'm sure many purists will think this is cheesy or cheating, but it just eliminates a lot of frustration if you've completed 8 of 9 laps in the lead, and then accidentally spin out near the end.
The car selection is quite decent... most of the "famous" cars are there (even Lambos and Ferraris!), as well as many human-attainable cars.
I had a lot of fun with the game until I got to about the last 25%... when I started feeling a bit bored. I felt like earning money became a chore near the end. When the higher end cars cost over $1 million, and you're earning maybe $60K total on a good race series, it was just too much "grinding" to afford the nice cars.
And final tip, when using any of the "racing" cars, turn traction control on, unless you enjoy constantly feathering the accelerator and spinning out.