I've done the "friends with benefits" thing a couple of times. Once was very brief with a man I'm still close to.
Once was quite longterm with someone I don't see anymore in any way. That one was a real learning experience. I learned that I really have to work hard to not let emotions get in the way. And I learned that he didn't.
Though it went on for a long time, I only have a few really vivid memories of the whole thing.
We made a bet once about something, and I lost. So I was his slave for a day. I had to wear a merry widow (I was much more "willowy" back then, so the idea didn't horrify me quite as much as it might now) and essentially do chores all day. I do remember his friends popping by for some geeky gaming thing, but if I remember correctly I was allowed to wear normal clothes at that point. I do remember that one friend was quite uncomfortable with me playing the geisha girl to them all. (Mmmm geisha...must remember that one...I wonder how cheap kimonos are...)
He was quite chatty. We could be in the midst of the most intense, vigorous sex and we be carrying on a conversation the whole time. His attitude was that sex was both interesting and ridiculous, and that people shouldn't take it so seriously. I agree 100%. Men who only want to grunt to gaze into your eyes the whole time? That's OK occasionally, but in the long-term? I think I would get annoyed with it. And heaven forbid that you laugh during! I had one boyfriend make me solemnly swear that I would never do so. Apparently he would lose his erection immeidately. Uhhh OK.
Anyways, sex with Vitamin D (his own nickname for himself) was quite phenomenal. I didn't enjoy being on top until I was with him. And I love love loved it when we were both sitting up facing each other, me riding him with him holding my hands behind my back and biting my nipples. No one had ever done it like that with me before or since, and it really worked for me.
(As a note, I do so enjoy having my wrists held down/tied up etc...something about the implied helplessness of the position....and since I have tiny wrists and hands, most men haven't had a problem doing this with only one hand, leaving their other hand free to explore me in other places...)
My only other really clear memory of this relationship was one night when there was a massive thunder storm. We were right in the throes of it, and most likely carrying on a conversation, when all of a sudden it sounded like lightening struck something right outside the window. I've never heard anything so loud. He was so startled that he slammed into me a lot harder than he intended. It was the most delicious pain I had ever felt....
Now of course this relationship evolved into more friendship, which culminated in the whole guilty-feeling "we probably shouldn't be doing this" thing. Things petered off after a while. But if given the question, I might still maintain that it was some of the best sex I've ever had...