auly: Yes, that smile was a YES.
Our experiences have been wonderful for the most part. It depends on how you view sex though. If it is viewed as a fun activity that turns you on and you can seperate the fun from intimacy, then it's cool. If you're in a ltr. and feelings develop between the three of you, there can be problems or it can be all good. It depends on the jealous natures between the people involved. If you're gonna be with the third person repeatedly, rules need to be laid out so that no one gets hurt. If the rules are broken (we're all human and sometimes we think with the little head if you know what I mean), forgiveness is a must unless you want the relationship to fail. If rules are broken repeatedly, then it's gotta come to an end to save the original relationship. Trust your gut instincts and your partner's. If one of you is getting feelings that something isn't going right, then even if the other disagrees, it needs to come to an end before things get out of control.
Very open communication is most important between the three people. If you have a lack of trust with your partner(s), then I'd avoid it all together. If you're in a casual relationship with someone, then by all means go for it at least once to see what it's like.
Since I'm bi, I enjoy being with a man and a woman. I love to see my husband happy like a child on Christmas morning (actually, that's what it's like for both of us...a gift to each other), 3-somes are just another dimension to adding some fun to our sex life. The expressions on our faces while the three of us are exhausted all cuddled up together just says it all.
There's nothing like having a "helper" too. When I'm getting fucked my my husband and our female helper is playing with other parts of my body while I'm getting to eat her out at the same time, it's just insane pleasure for all involved. The visuals are so hot! The possibilities are endless as to the pleasure you will all be experiencing.
But since I'm married, my husband and I only play with other women when we're both together, although if one of us is in a situation where we meet someone, it's ok to flirt to lay the groundwork.