Quote: "1) What is a reasonable learning curve for this expansion pack?"
It's very easy to use. It will take you some experience to learn the ins and outs of it, manipulating it specifically to your needs. No more than a couple days of tweaking the existing, and already working, settings.
Quote: "2) How easy is it to add to an already started game? (e.g. a car game, all made except for the AI)"
Very easy. You can turn the AI over to it totally. The hardest part for you will be to make all of your models animate correctly based on the AI.
However, it isn't appropriate for car AI. It is fantastic for the pedestrians and other characters. In my Dark Physics compo entry, I use Dark AI for the units and I wrote my own car AI for the vehicles.
Quote: "3) On the Dark AI info page, TGC states that it "comes with 10 example programs, each showcasing a different aspect of the Dark AI system". Firstly, what exactly do those programs do?"
They show the capabilities of DarkAI. It shows things like automatic AI control, teams, walking up stairs, etc. It's very comprehensive.
Quote: "4) Last of all, does Dark AI "take over" your game in the same sense that a physics library does?"
You can let it take over, or opt to control your units yourself. It's very flexible. I prefer automatic control. However, I use Dark Physics with it, so while I use automatic control, I have to manually control the characters at times. Very easy to do.
Bottom line - it is a fantastic addon for DBP. It was written by Sparky, and is done extraordinary efficiently. It taught me a lot about AI. If I make AI in another language, it will be based on his model.

Come see the WIP!