One of the issues I've always had with MCE2005 is that you can't use the VMR deinterlacing modes with the Purevideo decoder, you're limited to the standard "best available/bob/weave" options.
In Vista MCE, are the VMR modes now available? To check, play a DVD and open up the Purevideo settings from the taskbar icon and see what deinterlacing modes are available. I'd really like to know if this issue has been addressed before I commit to upgrade.
Kai