If you played the original Sega GT 2002, then you already know what Sega GT Online is going to be like. There have been a lot of changes between 2002 and the Online. The biggest being that Sega GT Online is Xbox Live compatible, giving you yet another reason to keep your subscription to Xbox Live.
There are more cars, the graphics are tweaked a bit, and there are more parts along with some time-triggered weather events. All in all, these minor improvements make the package all the more attractive. However, it's not an entirely new game. Personally, I would have liked to do more with the cars, in terms of performance tweaks and whatnot, but I will say that the online component really makes this game worth having.
First of all, you're no longer stuck with dealing with split-screen action for multiplayer games. You can have it all to yourself when you challenge others on Xbox Live. Second, the minor improvements do add up to making Sega GT Online an attractive package.
Overall, my opinion of Sega GT Online remains about the same as how I felt about Sega GT 2002. It's essentially Gran Turismo "Lite" for the Xbox. Needless to say, it provides a good amount of fun and adds another reason to keep or renew your Xbox Live subscription.