What do you think about non-monogamous relationships (see details)?

I’m not just asking about whether you would ever be one (though feel free to answer that question). I’m also wondering what you think about other people who are in consensually non-monogamous relationships of various kinds. And I don’t just mean people who haven’t decided to commit to one another yet. I am including longterm relationships that have various degrees of non-monogamy built into them (open marriages, polyamory, and all of that).