I had an ex back in my home town that I would probably see every other month, and end up fucking, we didn't talk often but occasionally caught up on things every now and then, there was no chance of us getting back together and we both seemed pretty happy with occasional sex. That being said, she got a boyfriend some time ago and completely cut off contact with me, whether that would mean there is residual feeling or whether she's just moved on I don't know. It can work if both parties are mature and honest I think, but its a very risky area, easier if avoided I would think.