It really depends on what you are trying to do. (like another said here)
For running mission critical systems. I would choose Linux or another Unix variant hands down! Mail, web, VPN, Samba, FTP type stuff.
What do I use on my desktop? Well I use WindowsXP at the moment. I could use Linux for desktop, but just have not found a compelling reason to do it yet. I have enough skill to keep my system out of trouble with viruses, network attacks, etc....
Now if the world starts to embrace Vista and all support stops for XP, that to me is a COMPELLING reason. I suspect Vista will go down in flames much like ME did. Just seems to be taking a little longer. IF and When "XP64" comes out, then maybe I will take another look at Microsoft's offering for the desktop.
I do mostly Office stuff and business programming. If I were a "gamer", then I would have to rethink my position.