I've updated my HTPC from Windows 7 with XBMC to Windows 8 with XBMC.
The one thing that bugged me about Windows 7 when I moved to XBMC from Media Center is that I couldn't use the Green Button on my remote to start XBMC reliably, even with changing various other things (registry settings, 3rd party programs etc)
So - when the £15 offer was on for Windows 8, I took the opportunity to get Windows 8, and have thrown XBMC on there.
I've cleared down TIFKAM so that it just shows 2 tiles currently - XBMC and Desktop.
I can navigate this with my MCE remote, and launch XBMC. I'll have to check what Add-On I've put on to allow me to use the green button and all the other remote keys within XBMC properly, if anything!
With the new XBMC (Frodo) coming out of beta soon, I'll give it a whirl - but for now I'm very happy with Windows 8 and XBMC - in fact, having Windows 8 on there has removed the one issue I was having with Windows 7 - Windows Media Center.