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.
The one that is increasingly getting my attention is solaris.