I had serious problems w/ XP64, especially with hardware drivers. The video drivers that came w/ my laptop for instance said they were 64 bit but I couldn't get them to install. It turns out they were just repackaged Southbridge drivers. I had to go to PlanetAMD64.com to try to find workarounds all the time.

Things improved as Windows Server became more popular but not enough. This is why I am glad for Vista64 because its the OS of choice and any new driver or application is designed with this version in mind first.

When W7 is released and people start picking it up like its Coke Classic or something I think we can expect more improvement and maybe more devs with will take advantage of multiple cores as well.