I've stayed friends with many exes, and re-dated several exes. I've even recycled a few exes by fixing them up with friends. Each situation is unique. If I had a chance to date an ex while in another relationship and he was okay with non-exclusivity, that's fine with me. I don't have any stories to tell, but I really don't see it as some earth-shattering thing, or any different from dating anyone, except for having a history together and memories to share - but everyone learns and grows and evolves as a person, so I would think that a new relationship with an old ex is simply a new relationship.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein