Improves on the GranTurismo family in a lot of cool ways! More cars! More races! Same great experience!
The depth of gameplay has been much improved, with special conditions races, mission races, tons of single-model races, and extreme events. All the old staples are back, including series races, endurance events and rally. I'm not exactly a guy that you would label as an absolute car freak, but even I love all the power!
Another characteristic that I've always loved about the Gran Turismo series is the actual race training that is included. I'm not saying that I could go out and win the Indy 500 or anything, but I know a hell of a lot more about competitive driving than I would have without Gran Turismo.
Drawbacks - Lack of car damage leads to use of sloppy tactics. Other than that, I think it's incredibly realistic.
Technically challenging and motivating enough to keep racing for a long time. If you like cars and racing games, there's not a better offering on the market.