I've played TT for a few hours, and even though I'm a bike nut, the game is a complete bore because of the non-existent AI. There's no sense of racing; you don't need to focus on good lines, or saving your tires, or stalking a faster rider because it's too, too easy.
I'm playing with pro mode on, and I didn't bother with golds on licenses cause I just wanted to do some competitive laps. A game like this helps me to appreciate Namco's MotoGP series, because they at least have the basic function of scalable difficulty, where you can set the game to easy, normal or hard. While I'd like to think PD will fix this in the PS3 version, I doubt it will, given how little they improved the AI from GT3 to GT4.
The physics are very good, but there's no challenge, and racing my ghost is just way too geeky (not that I haven't already crossed that line).
I can understand why there's so much emphasis on Photo Mode, because aside from admiring the pretty pictures, there's not much to do.