FWIW, I use FC6 as my primary desktop. It's quite stable. I wouldn't use it for a server however -- too fast of a moving target.
Why not? Fedora as a server is not a problem...
Fedora as a desktop however...I don't know...the few times I have seen Fedora Core 5/6 desktops in action, Firefox froze, keyboard input would not work all of a sudden...