Not sure where to start! Anything and everything mate, you get random AI collisions, cars stuck inside the track (not a typo), dreadful online system, cars pitting on the wrong tyres, it's just a mess. If it was one thing you could live with it, but it's loads of different bits. Will will tell you that when it's good it's absolutely immense, but you might get that hallelujah moment once every six hours of gameplay. A bunch of us used to play it religiously online, but since FM6 & Destiny TTK came out we've all ditched PCars.
If you can pick it up cheap, and I mean sub £20 cheap, then it's probably worth a go. Definitely don't pay full price though.
Dan has nailed it in his comment. When PCars is working as intended it is amazing, best driving experience i have ever had. But 80% of the time it is ruined by silly bugs. Sound issues and stuttering, awful frame rate issues on certain tracks, dodgy handling. It is such a shame as this could have been the perfect racing game. I know many of my friends uninstalled it after a day which is a huge shame. I have been told numerous fixes have been released but ive still had issues.
FM6 and destiny for me.