I just upgraded the HTPC to Windows 8 today.
I'm trying it out, though I cloned the system disk before doing so in case I want to bail after a week or so. So far, the upgrade experience was middling.
I actually upgraded the system in-place, which I haven't done since maybe Windows 95 > 98 (which was then later un-done). However, Paul Thurrott says you can do it, and I've read similar things elsewhere, so I'm trying that too. That experience has been mixed. It took forever. And, I had some issues... My graphics card drivers came through all hosed in multiple ways (which took some effort to diagnose and fix). That prevented MC from working right, and made Firefox crash immediately on launch. I also found some incompatible Shell item was preventing right-clicking in the Desktop mode entirely, which stunk (explorer.exe was crashing).