Of the responses so far, I agree with this one the most:
ehBeth wrote:Sometimes it works, sometimes it doesn't. The guidelines and results are as different as every couple is. It's definitely something to approach with extreme caution.
I don't know any 'numbers' on this, but the anecdotal evidence in my circle is very mixed.
One thing I was going to say was how would we respond to a topic that had the exact same title, minus "open"? There is simply no universal. Some work, some don't, and for lots of different reasons.
I understand that you're looking to get been there done that advice, which I don't have, and which is why I haven't said anything yet.
What I will say is that of the anecdotal evidence I'm aware of, it skews heavily toward reality being far different from the fantasy. Especially, no two partners seem to have exactly the same experience, and that seems to often be a source of friction. Wife loved it and REALLY wants to do it again, husband didn't like it much but was basically OK with it -- once was enough. Husband loved it and REALLY wants to do it again, wife didn't like it much but was basically OK with it -- once was enough. Wife loved it, husband absolutely hated it and is deeply disturbed that wife liked it so much. Husband loved it, wife absolutely hated it and is deeply disturbed that wife liked it so much. Both hated it, blame each other for making the pleasant fantasy into such an unpleasant reality. That of course leaves room for both loved it, too, but that seems to be the distinct minority.
Not even getting into the aspects of when extra-marital partners go bad -- diseases, jealousy on THEIR part, stalker, whatever.
What has struck me most though is how many people have said some variation of "I wish I could just go back in time..." It changes their relationship on a fundamental level, and not for the better.