I first installed Parallels under Tiger, a month before the release of Leopard, without any problem of any kind, and immediately after that did a custom install of Windows XP Pro, again without any problems, as far as Parallels was involved. It was, I believe, build 4428. I had only to familiarize myself with using Windows XP. I then asked (web form) Parallels support about compability with the approaching Leopard, and within a couple of hours, the support people recommended to upgrade to build 5160, and where from it could be dowloaded. Which I did. Then I upgraded from Tiger to Leopard, 10.5 (erase, clean install, restore). No problems, Parallels and Windows XP continued to perform satisfactorily. Updated Mac OSX to 10.5.1. I have never enabled Internet Sharing, I don't remember anything related to Internet Sharing during installation: I run LittleSnitch, which I believe would have reacted to any attempt by Parallels installer to contact its "home".
Parallels released a beta, I don't remember the build number. I consulted their support (web form), and was advised to wait for the final release of build 5582, which I did. Upgraded to 5582, still running fine, and in the meantime I managed to install Ubuntu 7.04 and to upgrade it to Ubuntu 7.10. At that time, I began having warning messages every time I tried to launch Windows XP, or Ubuntu, for that matter, related to a lack of available disk space. I explained the matter in detail (web form), waited a few days, no response, then I got one of Parallels RT notifications, informing me that "since I had not reacted to their answer, they considered the ticket solved, blah blah. I responded in very strong words that I had never received any kind of answer to my query. Less that 6 hours later I got both a detailed step step procedure to overcome the problem, and a short time later a response from someone else apologizing for the delay (that would be investigated, blah blah). That's what one could call lack of coordination. But the instructions of the first person worked fine, I commented back, got a final instruction, and that was it.
So maybe I was lucky, both in installing and upgrading Parallels, and in getting response from their support people. And I shall not pretend that everything was just dandy. There is something that I cannot understand with Parallels "support team": they seem to have very capable people, and they seem to have less capable people, to put it mildly. From what I have read in these forums, and not only in this thread, support's average is far, far below par. It shouldn't be. Parallel's products don't come cheap, 30 bucks for telephone support is highway robbery especially when you don't get anything for your 30 bucks. I hope that when I decided to go for Parallels, at the beginning, I didn't bet for the wrong horse.