Largely positive experience thus far: my current relationship is technically poly, though so far it's mostly been limited to when one of us is out of town for a week or more. General rule is "anything goes as long as you're safe and it doesn't cut into the time we have with each other." Travel (I'm a consultant) is one obvious case, but we live far enough apart that it's hard to see each other except on weekends. She's had one fling with an old fuckbuddy mid-week while we were both in our respective hometowns; I so far have not (only when traveling) but have considered it.

When we are with each other we have largely acted monogamous so far, and things may change if she moves closer (as she is planning to do). We have discussed (her suggestion) adding a third person for a fling, though. We appear to have implicitly rejected expanding the primary circle. Current relationship age is over seven months and this has been the rule since the first month.