I've spoken on this issue in the past. I was with my partner for almost 23 years prior to his sudden passing, and we had an open relationship... My idea originally and he eventually conceded. It was just easier than sneaking around and cheating, and the open, honest conversation made our comittiment to each other stronger. On occasion we played together, I would often watch him with other guys and it was a huge turn-on. While I was never into anal, I gave him the gift of enjoying that with other men.
Neither of us was the jealous type and I believe that is an important factor in considering open relationships. If your feelings will get in the way, it isn't gonna work.... Each couple needs to come up with their own guidelines and comfort zone for such a relationship...