If you like the S2000 based cars then yes, the cars are a great addition & the tracks are brilliant for them.
If you want to Camaros then yes again, they are good too and again the tracks.
Otherwise you may as well not buy it, but then again it's worth it just for the tracks IMO
The new tracks are interesting and tend to be shorter and tighter than the RACE07 tracks. I was keen to try the Mercedes C-class (BMS) but find it very oversteery and just too much of a handful for me. I am trying to tweak the setup to calm it down a bit.
Okay, I have decided to buy it after reading these good reviews. However, it is sad that there is not so many online servers running STCC. But again, this is what RaceDepartment and other SimRacing websites are for - making people come together and enjoy almost exactly what they want to.