My husband and I have opened up, and it's been a net improvement to our relationship. We've been married 10 years and it's just made everything better from our communication to our sex lives even when it's just us.
To echo the sentiments here, communication is paramount. As others have said, too, having a clear understanding for YOURSELF of what is off-limits or potentially upsetting for you is important, as well as communicating that with your partner.
One thing that doesn't seem to get addressed often enough, I think, is this:
Sometimes you don't know that something needs to be a "rule" or renegotiated or clarified until after it's upset one of you. You MUST be prepared to deal with that, because sometimes it sounds simple when you're first talking about it, but relationships and feelings are complicated enough even before being open. You WILL get upset at something he does, and he WILL get upset at something you do -- not because there was malice involved but because "You don't know what you don't know." I don't know of a single open or semi-open couple who haven't had that happen and haven't had to renegotiate or clarify a rule midstream.
And that's okay!
How you handle it after the fact, with all the complication of emotions, is what will make all the difference in your success. Be open, be vulnerable, and be honest with yourself as well as your partner. Sometimes watching your partner have sex with someone is super hot, but hearing about them having sex with someone because you're not there can activate all kinds of insecurities you thought you had in check. Really ask yourself what it's about and be okay with the fact that it's a little bit of work.
Talk and be honest. Don't sit in anger or hurt. Don't pretend things are fine if they're not. If you're "going through it" and need to sort out what you're feeling before you talk about it, say so.
