Too many, really, to pick one. Katrina and I as very different. The core lesson for me was that while it's easy to pair with someone who is like you, it's better, if you can stand it, to pair with someone who is different.
Being forced to resolve our differences improved our understanding of the material, and vastly improved that book. This only worked because, despite our head-banging, we respected one another's ideas, and believed that if we stayed at a discussion (read, argument) long enough, we would find that we actually agreed.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.