My biggest issue is that the AI continues to punt me off track when I slow down. In most cases, I'm slowing down because of traffic in front of me but the AI behind me won't adjust. A perfect example is the first chicane at Monza. In the beginning lap of a Monza race, it's almost guaranteed that I will get rammed from behind. Another example is the Nurburgring Nordschleife when there are a large group of cars racing together. It's like they are programmed to take corners at whatever speed regardless of what is in front of them. They do however, seem to be good in side by side situations though. Going 2 wide in turns will normally result in them giving up the position or at least maintaining their line; I like that, make me work for the position.
Outside of the punting, I can deal with the rest of their behavior for the most part. Them going too hot into turns is natural...just brake earlier and perform an over-under maneuver. When they come back on track, I give them room like I would a live player. If I see a can in front that is driving a bit erratic, I drop back a bit and wait for them to mess up...again, like I would a real player.
Good AI is important for me because the majority of my racing is done offline. When I come home from work, I don't always feel like finding a race, qualifying then racing. I just want to get in a few good races before I wind down for the evening. Most of my online racing is done on the weekends, if I don't have plans. And to be honest, the online races that I've run weren't much better than my AI races and in a lot of cases, they were worse. So having decent AI is a big part in determining which sims I drive the most.
I think Kunos is making good progress, I've certainly seen improvements in the time that I've owned A/C.