Well, where to start...
I live with my GF and her 5 year old son. While we were dating, I didn't spend a lot of time with him because we wanted to "be sure" before allowing him and I to connect. When I did see him, he was an angel, which now I know when he is around strangers is just about the only time he behaves. So when I decided to move in, I had no idea the level of difficulty that was about to ensue.
My GF and I get along great; she is my best friend, and the only arguments we ever have are about her son.