I'm doing it currently. It's working out well for us, but I think that's because of the special situation.
Specifically, that ex and I broke up because of outside influences in the relationship (mainly, issues between me and another partner) and not because there was anything not working in the relationship between the two of us. We've also both independently thought through that situation and dealt with the ways that our choices were the best possible option in a shitty situation, so there isn't any animosity or resentment or even old issues from our previous relationship to address.
Also, it's a relationship I was in before and during my current marriage, which makes it the ultimate unthreatening situation for my husband -- we have literally already been there, done that and bought the tee-shirt.
Personally, I don't think anything else would have brought me back to polyamory. We'd been monogamous by default for so long, and because of how busy we are with family and career stuff had long switched over to not having relationships with people who happened through our lives just because we could, but really waiting for a deep connection. We're a bad bet on several fronts anyway (we move a lot, we have small children and crazy work schedules), so as much as I wanted that connection, I'd admitted it just wasn't likely to come together unless I was willing to make a lot of pretty fundamental changes in my life. Being lucky enough to have the opportunity to reconnect, especially with someone who I have such deep feelings with, is an enormous gift and I feel unbelievably blessed and lucky.