I just got the game two days ago and it has become an addiction already...
The car selection is immense, as is the variety of upgrades and body mods. The cars aren't named exactly what they are, but they have a code and that code translates to a real car (all faithfully sculpted). You can get the list at gamefaqs.
Paint jobs (another one of the most common "gimme!" items of GT3 players) are available and easy to use.
I actually like the AI *more* than GT3. Rivals vary wildly in skill and aggressiveness, which is a nice change from "everyone follows the same line and uses same braking points" that dominates GT3.
This game enforces clean driving a lot more than GT3 does, as the races are often too short to make up for a bad mistake. The problem that GT3 has in the Pro League is, therefore, removed (races can't be won on tires alone).
It isn't as pretty as GT3 (though it is far and away better than GT2... I can't believe that someone said that earlier) but it is adequate and I am very likely to hate a game if it looks bad or even blah. The cars aren't named, but they already provided a means to rename them appropriately. While there are more modifications, there are less tweaks that you can make in TXR0 than in GT3 (but the important ones are still there).
Anyway, in case you can't tell, I'm already a huge fan of the game. I'm ashamed I put off buying it for so long fearing that it just wouldn't be as fun as GT3. If you get a chance, it's only $20 new (probably $10 or so used) so give it a shot. If you'd rather be racing modified "normal" cars than F1 or GT racing cars, then this is the game for you.