Haha some of the posts here are really funny.
I use it because it's actually my OS of choice. I am knowledgeable enough in mac and linux related stuff(my first PC was a mac btw, albeit a realllyyy old one lolz, mighta been an Apple II XD), and have used the three main OSs extensively, at one point Ubuntu being my main OS for months.
For me, windows is the most balanced. I wish everything were open-source and free and full of unicorns and rainbows, but right now I can't see linux yielding widespread usage.
OSX for me is just too... Idk. I remember reading an article about how the iphone being Simple to use didn't necesarily mean easy to use, and I find that kinda applies to Macs. Whatever the arguments may be, I see absolutely no point in having a single mouse button(yea of course you can simulate right clicks or install your own mouse, but still), or the reason for a separate control and option button. Some simple functionality seems to be sacrificed for the sake of simplicity and minimalism, which is a no-no in my book. Also, gaming. I'm not heavy into gaming now, but I was before, and unless I buy a mac pro customized and all, I'll never get the ebst gaming experience on a mac.
Linux is still too complicated for me. I know plenty about how to use it, but frankly I don't like using terminals for actions I should be able to do simply with some clicks. I am a heavy mouse-oriented user, and rarely use keyboard shortcuts and avoid using the keyboard whenever I can, so yea. Also, as customized as I had my install of Ubuntu, I don't know if I will ever get that truly "polished" feel from Linux. It feels like a well made OS by smart peeps, but it just lacks that polish I find that Windows and especially mac have.
So windows strikes the perfect balance for me. marketshare and compatibility nonwithstanding, it is truly is my favorite OS. For me, it's the one that "just works", usually. I liked vista, but prefered Ubuntu. But of course, now Wndows 7 holds my hard drive
