Negative experience, I know all the people involved but didn't observe any of this firsthand and it happened before I knew some of them: A good friend of mine (call her person B, female, bi, married) and her husband (person C) once tried dating another married couple (also friends of mine, call the guy person D). Everything went swimmingly as far as the sex and the hanging out together as friends went, but C got uncomfortable about the growing romantic attachments and amicably broke off the inter-couple relationship. Unfortunately, B had already fallen hard for D (though not to the exclusion of C) and ended up cheating to have one more night with him. When she told C about it he got pretty mad, blocked D out of his life (and got a promise from B not to be alone with him again) and pretty much swore off polyamory (at the time). This was over three years ago, and it was only in the last year or so that C has started to forgive D and they've moved towards being friends again.

B and C are still married, though it was rocky for a while there (D and his wife aren't, for many reasons of which this cheating incident was plausibly one). B really doesn't do monogamy well, and the compromise for a while was swinging with other couples (just sex, no dating in the usual sense) every now and then. That seems to have worked out, though B wishes it was more often.

Related question: should I include swingers in the list of people I know in poly relationships? The boundary is a bit fuzzy but many people would count it as CNM even if it's just for sex.