I did it for about 2 and a half years. It was a lot of fun for most of the time we were together and there was nothing about the experience that wouldn't make me want to do it again.
My ATF and I discussed getting together and getting into an arrangement but we decided that if we did, it might change the relationship that we had, and that we still had, and we were both really happy with where we were at. I think that was a good long term decision for me because I find when we're together too often, I find myself falling for her in a way she doesn't want to be fallen for.