I've been cheated on. It destroyed our marriage (of 8 years), hurt my daughter, put me deeply in debt, wrecked my faith and self esteem, and even made me give up my dog (damn, I liked that dog!) So, no, can't say that I have anything too positive to say about affairs. It can destroy your life and those around you.
It was the deceit of it all that hurt me. I don't have a problem with "open" relationships, because it is out in the open, with your partner's input and blessing. Not saying I would feel comfortable in a relationship like that myself, but I think it is fine for those who do.