Personally I use windows 7 because it is easy to use and is the platform I use at work. Furthermore this machine is the home computer which I share with my nephew and sister, and I doubt they know how to use anything else.
But recently I decided to try Linux and installed Mint 15 for my personal use. I love how fast the machine feels running linux, though I had a pretty hard time configuring the hardware.
I decided to use Linux because I do not want to entirely devote myself to Windows, as a programmer I want a more flexible platform to experiment and learn.