Personally: Overall positive experiences. I'm polyamorous by nature, and have never had a relationship that wasn't poly. In my friend circle (bay area rationalists) there's a fair bit of polyamory. It seems like there's more + happier relationships, as well as more + calmer breakups, when I compare to the current relationships of my acquaintances from high school.

Negative data point: someone I know tried polyamory for (I think) 10-25 years, had a lot of difficult life experiences some of which related to her relationships, and has lately skewed towards relationship anarchy but with one primary romantic partner.

Data point in favor of poly, but sad: I know a person who left a 10-year relationship last year due to (her own) cheating and has been cheerfully doing CNM since then.