I can only say that, for my husband and I, it has been a wonderfully fulfilling and enlivening experience and has made our marriage even stronger and more satisfying.
We realise that such an "open relationship" wouldn't work for everyone . . . but I do actually feel that the honesty of expressing our innermost desires and fantasies just brought us closer together. It has strengthened our love and commitment in so many more ways than just the obvious sexual side of our relationship.
From many emails I receive, and even from the experiences of several of my friends, I think that one of the main reasons people enter into affairs is because they feel they can't 'open-up' enough with their own partner, and consequently feel neglected and taken for granted. If only more people were able to share those emotional (and physical) desires that we all have from time to time, with their spouse, I am sure that many, many more marriages could be enrichened as ours has.
I know, that for some, our lifestyle is considered immoral and distasteful . . . but it has made our (almost) 30 year relationship even more exciting and wonderful with every passing year.
So, in my view, the answer to your question is: "It CAN be GOOD in the right circumstances, and for the right people".