I have been able to do it only once (currently). Most of the time there was a sensation of something missing. I was, in fact, surprised when it worked out this time, and it almost didn't--we found a lot more common interests, including professional collaboration, but it was still much more intimate and personal than my other frienships. Most of my most enduring friendships are with people I never had sex with, though. I also tried to be friends with an ex-girlfriend many times, but gave up after years. It had less to do with me than her in this case: All of her friends put up with the fact that she likes to fight very frequently (with friends and lovers alike), and this I can't abide in a friend. Oh yes, there was, in fact, another male lover that I didn't see for perhaps 12 years after we broke up; we were then friends for another 8 years until he died unexpectedly, so I guess it's happened more than once with me, and I can even think of one more, but I've lost contact with him and have been unable to find out what happened to him.
However, looking back over the thread, I do see that I wouldn't be able to be friends with someone that I still mainly wanted to have sex with. It can be sustained only after there was a cessation of the sexual need. If I still want the sex and the other doesn't...well, there's nothing there to work with, is there, except frustration of both, one for not getting sex, the other for being pressured.