I did have an open relationship, but in retrospect, it was probably just an excuse for my ex to go and shag other guys without me, because she was significantly more attractive than me and being a woman, it was easier for her as well.
During the brief time when we were "open", I think she had 3 regular outside partners and I had 0 (besides her). That pretty much started the breakup - she fell in love with her lifestyle, said we really could only stay together if she was allowed to continue.
Anyway, while we were open, I met quite a few of her women friends who encouraged her to do this also; I found that while there were quite a few couples who claimed success and that there were no problems of jealousy, I don't think they were able to do this for longer than a few years. I do keep in touch with some of them as mutual friends, and while nearly all of them are still in open relationships, almost all of them have changed some part of their partners since then, so it's kind of hard to say whether this lifestyle is "stable". They still seem happy for their lifestyle choice, but then, as one woman told me, she found being dependent on one person for her happiness terrifying and that alone made being with one person worse than being single. Go figure.
Anyway, to each his own I guess. By the way, I'm now happily married and monogamous. Feels right to me.