I am not a giant fan of the GT series. My roommate played it obsessively and I was always sort of interested in playing it, but I never could. It was tedious and lame.
I love Forza 2. Like, insanely love it. I can't stop playing it. I have to play Shadowrun before Forza because I know if I play Forza first, I'll be playing it until I have to go to sleep.
Best simulation racing game ever? Very likely.
My heavily upgraded Mk2 Golf makes grown men cry because it is so fast.