So, in terms of statistics I'm not going to try to isolate the experiences; I have too many friends involved in poly relationships to do a representative sampling without more work than I feel like dedicating to this comment.
Just to be clear, I'm counting as "poly" relationships those where at least one partner has at least two partners who know about and are reasonably OK with each other for at least some period of time, and where such partners are understood as in-principle acceptable. I'm counting as "monogamous" relationships where such partners are understood as in-principle unacceptable, including ones where partners have had affairs but claim to feel bad about them.
This creates an excluded middle of "poly in principle but not in practice" and it's likely that a lot of relationships I consider monogamous fall in that excluded middle (arguably including my own) but I don't really care.
There's also "nominally monogamous but in practice poly" relationships mediated by affairs which are eventually confessed and forgiven, about which I don't have much to say but am counting here as monogamous.
If I had to guess about statistics I'd say that maybe 15-30% of the long-term (say, a decade or more) monogamous relationships in my cohort have tried the poly thing and concluded it isn't for them, and about the same proportion of the LTRs in my cohort are poly and seem happy enough with it. (Which is not to say they don't have relationship problems; they do, sometimes involving third parties and sometimes not. So do the monogamous LTRs, and the single people.)
I haven't noticed either groups' relationships failing more often, though I see a lot of relationships of both sorts break up on third parties, which in poly relationships usually looks like a triad destabilizing, and in mono relationships usually looks like an unforgiven affair.
One highlight: one of the best marriages I know of (in terms of how well the spouses work together and support each other and make each other happy and achieve their own goals and raise their child) has been poly for about a decade now; they've each had secondary partners during that time who last a few months to a couple of years and then break off amicably.
Related article: Polyhacking
Note: This article was posted earlier for less than a day but accidentally deleted.
Although polyamory isn't one of the "official" topics of LW interest (human cognition, AI, probability, etc...), this is the only community I'm part of where I expect a sufficiently high number of members to have experience with it to give useful feedback.
If you go looking for advice or articles about polyamory on the internet, you mostly get stuff written by polyamorists that are happy with their decisions. Is this selection bias? Where are the people whose relationships (or social lives, out anything) got damaged or ruined by experimenting with Consensual Non-Monogamy?
I'm posting this hoping for feedback, negative AND positive, on experiences with polyamory. I considered putting this in an Open Thread, but it occurred to me that many other LW readers might be interested in whether polyamory has drawbacks they need to be aware of. If you have experience with CNM (including first-hand witnessing, which has the added bonus of not requiring you to out yourself while still participating in the dialogue), please comment with your overall impression and as much detail as you would like to include (I am also putting my experiences there rather than in this post). If you've seen multiple poly relationships, multiple comments would make tallying slightly easier. I will try to upvote people who feed me data, a la LW surveys. If there are sufficient comments, I will periodically go through them and post a rough ratio of good to bad experiences at the bottom of this article.
PSA: The Username account is available for use by any who wish to remain anonymous. The password is left as an exercise for the reader. Hat tip... Username.