A close friend of mine did that. He was in a Ph.D. program at Santa Cruz. His gf decided she wanted to get more serious with one of her partners and left him. Then he seriously flipped out

I mean really, really lost it. He left school and wandered around the country. I still don't know if he's ok

I'm not saying the open relationship caused that, but the issue with his gf was the major catalyst for his freak out.
I personally would not be into such a thing. It just opens the door for all sorts of problems. I don't normally think of myself as the jealous type, but I think alliances and preferences would form, and the uncertainty would lead to instabilities.