Now that we're here, I just wanted to say that this game holds up really, really well. I guess it's even more striking to me now that Dirt is out, which was the first game that could have seriously challenged it for me. For now, it's still one of the finest online racing games, especially with it's 12 player support and awesome physics (Dirt is really a let-down in that respect) and alternate routes and different vehicle types.
GT5 Prologue will probably be the first game that can challenge it in that respect, although of course they are very different experiences.