I just upgraded my Elonex Artisan Media Centre from Vista to Windows 8 (with Media Centre pack).
The upgrade went surprisingly smoothly, once I had added the old sound drivers, and forced Windows 8 to use my old X300 video drivers.
I thought at first the machine was going to be unusably slow, but it turned out the indexing services were using 90% cpu, and after an hour or so the machine became more responsive.
However, I now have a problem. If I press the power button on my Media Centre infra red remote control, the machine appears to shut down (all the lights go out, and it does not respond if I press the remote power button again). If I press the power button on the front of the computer, it starts up again, and goes back to exactly where I left off (perhaps it is hibernating - but it seems awful quick).
If I press Win+I, then sleep from the power menu, it goes to sleep liked it used to - i.e. some of the lights stay on, and the remote will power it up again.
You can also choose Sleep from the Media Centre menus, and that sleeps properly too. It is only the Power button on the remote that shuts the machine down (or Hibernates it - I'm not sure - whatever it does, the machine is powered off, and will not restart from the remote).