I will say for now just wait and play something else. I haven't tried MP yet but AI is quite disappointing for me. I watched an old review by Empty Box and he praised AI, so I preordered the game, believing that "even after MP of this game is dead in the future, I could still have nice SP game and eye candies". But I just tried Formula A at Imola and Superkart250 at Road America with 100% AI, and the experience was quite bad.
AI crash way too often, and I have never seen so many car flips in any game. Even when the AI is positioned #1, it can take a corner in a manner way too aggressive and flip itself. Sometimes I even feel that I am playing NFS:MW. I'm a slow driver, and I could barely keep up with 85% AI of GSC and AC, but at least for Superkart250 in PCars, 100% AI is way too slow. I wish they could fix this problem soon.
The game does look gorgeous at Ultra graphics settings. Also it runs quite well. I have a GTX770, and I could turn almost everything to Highest possible (anti-aliasing is too much for my computer), and still have 30FPS in thunderstorm and 60FPS in clear at 1920*1080 while recording a video using bandicam. With everything on low, I could run triple monitor at over 100FPS while playing in clear, and when I record video with bandicam I could reach over 90FPS.
The game's good looking and fun, and I would definitely recommend it to any of my friends. However if you are asking whether you should buy it now at full price (can people still get those bonus cars now?), I don't think that's a particularly good deal at the moment. If you have something else to play and you are still enjoying it, keep doing that and come back after PCars get more improved.