It depends on the situation, really. I am friends with my most recent ex, before my husband, but that is because we have a child together. It takes very understanding people to remain friends after a breakup. Jimmy, which is the ex, and I have a really great friendship, we were friends before we were together and we'll be friends for the rest of our lives, I'm sure. He's actually my best friend, after Rob, my husband, of course!
