Using Vista Home Premium. Perfect so far. The only niggle is constantly being asked permission to run apps.
I've had no issues with getting codecs for 64-bit Vista
CoreAVC works a treat for instance. Media Player codecs also work as they should, as does DivX/XviD.
I've also got Corel WinDVD working lovely.
I try to follow the 'less is more' rule with codecs and only install what's essential as and when it's needed and I've not experienced any trouble.
I have had a few niggles with Vista64 and codecs, for example ffshow64 does not work with my Vista64, but ffdsow 32bit does fine.
But i have to stress the niggles are just that, niggles; very small issues that can be easily resolved.
I dont know how much better 64bit OS works with a 64bit capable CPU though...anyone?