I think it's an interesting feature, but it's still something that is a "once in a long while" feature. And, when I do use it, it's often pointless since I will need to take it to the mechanic anyway. If this was a free feature on my car, sure, I'll use it, but I'm not going to buy a product for that feature. More importantly, people who really don't care about cars in general and just take it in when the lights go on aren't going to either. (Especially when you consider the cost of getting your own self-diagnosis wrong. How is my mom supposed to know if a loose gas cap is a big deal or not?)
I think there's a lot of potential here but it needs to be something that a) I will use all the time and b) I will get real, tangible value out of it, not academic understanding of my driving habits or a few bucks a month off of my gas by changing them. (Which, by the way, is another "thing" I now have to worry about: is Automotive telling me my driving is ok? What do I need to change? Now I feel like a loser when I don't "do well" one month.) It needs to make my life easier or allow me to do things behind the wheel that are valuable but currently hard/dangerous to do. I think there's plenty there.