I did the Windows 8 upgrade to my workhorse Vaio Z3 (short for Vaio, Z Series, 3rd Generation) today. My experiences from testing with the Asus Vivobook had been mostly positive, and as I had started getting bluescreens to Windows 7 in Vaio, it crashed if it went to sleep mode, the PMD discrete Radeon card was no longer recognised, etc., it would had been necessary to reinstall its OS in any case.
It was necessary to unplug the PMD during the installation, as that peripheral meant that Windows 8 would get stuck into the adding devices phase. And that meant borrowing another external DVD drive to access the installation media from, but otherwise the process was pretty straightforward. Some drivers had to be removed prior to installation and loads of drivers for Windows 8 were available from the Sony support page for Windows 8 upgrade. The only thing missing was the display driver for the hybrid Radeon/Intel display system Vaio Z3 is using. I try to post the link for that later, it is an unofficial beta thing, but seems to work fine with the PMD.