I've never really used Linux as a "desktop" system. Instead it has always been a "server" where I would run web servers, database servers, file servers, etc. With the new box, I've actually connected it to my monitor using a KVM switch so for the very first time it can potentially be used as a desktop system.
Basic things such as browsing with Firefox, etc. worked out-of-the-box on Fedora Core 5, but one area that I've never used Linux for was to play video and sound. I downloaded the VLC player from rpm.livna.org (which also hosts RPM packages for ATI and Nvidia video drivers) to see how well it can play some of the media files that I have. The files played without any problems, so I was very pleased, but I haven't tried it on any Windows media files which I heard did have problem on 64bit linux.