Did it once, a long, long time ago. Won't go into the emotional aspects of it, but as far as sex goes, we had a very high phone bill considering all we could hear of one another was heavy breathing.
If I were to do it again, I'd make sure that we had a webcam hook-up in operation.
Communication is the key. It's so easy with IM, email, webcamming, etc. But don't forget the handwritten letter. A couple pages written by you is much more romantic than any email.