I don't like them, I don't get the point in them to be honest. It's just an easy way to have someone, but shag around and not get into trouble, well thats how I see it anyway.
I would never have one, if i'm going to be in a relationship, it's between me and that one person. I'm not going to share them with anyone else and I wouldn't even consider letting anyone else touch me. To me, if you are going to be in some kind of "relationship" then you should commit to them.
I don't think they can work, not really. It's too easy for one person to change their mind and back out of the "open relationship" because their feelings are getting stronger and they no longer want to share that person with anyone else.
If people want to do it, fine I don't really care but to put it bluntly, I think it's quite stupid.