Second this! Unlike GTS, You can have some really competitive races against the AI in ACC. Worlds different than GTS!
When I started playing ACC regularly, I tried to race the AI at AI 100 Skill/100 Aggressive. That was too much. I had to back it down to 90/90 until I got a good feel for ACC and then started upping the skill/aggressiveness. I've been playing an hour or two everyday for 2 months now and I'm only up to 93/93 and I'm getting some great races against the AI at my skill level right now. As my skill and feel for ACC improves, I can up the skill/aggressiveness of the AI. I like that better than the "professional" level of AI in GTS that is SO SLOW and far from competitive.
For me, the upside to racing the AI in ACC is that it is competitive, I've stopped worrying about dive bombs and bad racing in general and my driving/racing skills have improved. All of that and I don't have to race other humans for a competitive race. I'll take it!