The AI is definitely overly aggressive on higher levels.
It's gotten to the point that spending some money on repairs at the end of each race is now the norm rather than the exception, because some of the CPU opponents really lack racing etiquette. I don't mind that too much, though, as contact plays a large role in closed wheel racing.
I'm also hating the surprise braking that takes place on some very easy turns, like the last turn at Infineon. I always make sure to keep my distance on turns that require braking, to allow for some early braking on the CPU's part, but it angers me when I slam into the car in front because the CPU was braking for a turn that can be taken flat out or close to that. Sometimes the CPU will even brake on straights. COME ON.