I'm not sure this is the best forum to ask that question - it's been posed a few times and generally gets pretty critical responses...
As you asked about personal experience I can say it seems to work for us - together 17 years, married 13. Open for about the last 7 or 8. Happy.
Setting 'rules' up front is very important - what is and is not acceptable to each of you - and being honest about it. If there is something you are uncomfortable with, let your partner know either beforehand or immediately and discuss. It takes a while to get a balance I guess that you're both OK with, and it may be that in reality it's better not to go down that route.
Do not feel pressured to any extent - as some have pointed out, it's often 1 partner pushing more for it than the other.
For those who said why get married if you want to do this - I guess it depends on whether you see sex as necessarily something you want to limit to one person, or whether it is more the emotional side that is important. Our view was that we could separate sex from love and that it would not damage the relationship - so far we've been proved right.
Having said that - like all things, it develops over time - it's been a while since anything has happened outside our marriage now and we both seem content with that (getting old?!) so it's far from the overriding ingredient in our relationship.
Also, for those of you who may think it's always the man pushing for this, ours was mutual and the reality is it's a lot easier for a married woman to find someone who will accept this outside the marriage than the reverse. My wife has certainly been more 'successful' than me...
If anyone thinks I'm a hairy handed troll, have been on here for years and not name changed.