My longest-term metamour is Lora, my partner Jon's ex. I think she has dependent personality disorder, and she was verbally and emotionally abusive to Jon. They broke up earlier this year.
My relationship with her started out hopeful, went downhill, went back uphill after a huge throwdown around Christmas 2013, where she admitted that she'd been being really awful and promised to treat Jon better and start getting to know me (she'd been saying that she wanted to get to know me for months, but admitted that she didn't want to, as she was convinced I was out to get Jon, so had been doing her level best to avoid me while proclaiming to want to be my friend), and then went back downhill again last fall when she started being increasingly abusive, controlling and dependent, ending in Jon breaking up with her. By the time Jon broke up with her, I would only speak to her about factual things (like if we had any toilet paper, or where the kitty litter was - we lived together at the time). We were going to have to cease living together, I'd decided. Had Jon continued to date her, we'd have had no contact for at least six months, probably longer, as she got help for her problems (in theory. even though she's still in therapy, I don't see much in the way of signs of her really admitting to having the levels of problems she has or working on herself. she'd rather blame other people).
I really hope to never have another metamour like her again.
My metamour Jared used to be someone who I dated. I dated him and his wife Issi together for about a year and a half, then (at my behest) Jared and I transitioned to a friendship. Issi and I are still dating. He's super-awesome. We have a great relationship, and we're...honestly, probably we're friends first, metamours second. Issi and I have been friends for years, and Jared and I got to be friends about five years ago, so we were friends before lovers, and we're now just better friends than we were before we dated. If I had more time and energy, I'd love to spend more time with him one on one, hanging out, because he is such a cool guy. I really enjoy the emailing that we do back and forth, and I love seeing him whenever I do see him.
I guess I sort of have a third metamour, my love/friend Aaron's wife. Aaron and I are more than FWB, but less than actual dating partners. We live on opposite sides of the country, and don't get nearly enough time to talk/email/etc (one of those "in a perfect world, we'd be so much closer than we are" situations). Aaron and his wife don't quite have a DADT, but she doesn't really have any interest whatesoever in knowing details about his lovers or knowing them at all, so I don't know her at all. That feels kinda weird to me, and I think that, were he and I closer, it would bother me. But as we're not (and there's little chance we would be closer, at least not for years), it's OK the way it is.