Opining... you can do meaningful things with it provided AI races, particularly AI races on an oval, aren't an interest of yours. A lack of yellow flags means oval AI have zero sense and will just pile up as soon as one wipes out. Road course AI are quite robotic and often cut corners on the first lap while they try to maintain side-by-side conditions separated by more than a car width. The AI, however, are more fun to race with than AC's AI because AC's AI do some really random acts (like they will take you out from behind if you trap them on the wrong side of the racing line rather than blend in behind you).
The made-up car classes mostly make sense, though you'll find one car might not really fit with the others in a particular class.
The DLC is worthwhile if you have an interest in the specific content. For myself, since I'm not interested in rallycross or drifting, the Fun Pack was uninteresting save for the '66 Mustang. Similarly, the Japanese Cars pack was saved by the 280ZX and Group A R32 since I'm not interested in trackday cars or concept cars. Porsche Pack is a must-have and likely the upcoming LeMans Pack is, too.