Microsoft, well, I am not sure why I even want to use it except as a means to support a device that I cannot manage to get a Linux or BSD box to handle (a number of manufacturers are still poor at providing Linux device drivers), or if I need an application such as optical character recognition where open source software is weak --- GOCR and OCRAD are just not as good as the PC software out there.
But just for web browsing and email, aesthetically I think Firefox, Thunderbird, and Evolution on X now look better than on Windows, and certainly I find it pleasant to not be worrying about the enormously proliferated malware infecting so much of the Windows world.
I've been using Red Hat 9 (which is a couple of years old) on one computer recently and Firefox is noticably faster on Linux than on W2K. The lack of software is a bother, though, especially something like PhotoShop (GIMP does a lot but is harder to use than PS).
The multiplicity of sources for Linux probably means greater stability of design as there isn't a single source who can change everything at will.
ping
Because they are too cheap to buy a MAC.
Reason I switched to Linux:
I like my computer to be working, rather than to be working on my computer.
My uptime record is 62 days without a restart, and then, I only had to reboot because of a power failure.