Bought it too, last week. Whatever you call it, it's not GranTurismo. It's based on the same gameplay, although simplified, the circuits are the same (although less) except for Valencia (not present in the GT series, IIRC), there are a lot less bikes than there are cars in GT4, it's got less events. So it isn't GT for bikes. If you, like I did, come from a fortnight of playing GT4 and then take TT, you'll be disappointed when you look at the main menu in Tourist Trophy mode.
Is it worth it, though? Hell yeah!
It's got it's niggles, lacks many things, the bike selection isn't as compreehensive as it should be (lots of market leaders left out, for instance), yada yada yada. But I haven't been able to put it down yet.
I can't compare it with others though, since it's only my second PS2 game, and the first was (you guessed it) GT4.
Now for the 'Ring... f***ing awesome.
I can't help but end any gaming session with a lap or three around the 'Ring. My current best time is 7.58.8xx with a Ducati 999R (in full road spec), closely followed by a 8.02.8xx in a BMW K1200R (road spec too).
But I'm still working on it...