My big problem with it is that there's still plenty of things flat-out broken (tire wear in the so-called "endurance" races, for example), plus all this content like the Vision GT cars that we were promised, and they've done pretty much absolutely nothing about any of it since launch. I played through the game quite a bit, did everything I wanted, and started waiting for the fixes and new stuff that never came. So I just don't play it any more. I think that's where the game is right now. Not that nobody bought it, it's just we all stopped caring a few months ago.
As for pure sales numbers, they did zero marketing for the game. It showed up in stores and no one noticed. Hell, the day I bought it, I had to ask an employee about it, and he seemed surprised to find out that it released that day. And he had to go into the back to get a copy... they didn't even bother putting the game on the shelf on release day. That's how much marketing Sony did.