People will use whatever OS they feel comfortable with. For them, that is the best OS, and rightly so.
Both of my home systems have Windows operating systems. My wife uses Vista since I gladly handed it down to her and I use Windows 7 x64. All of my PCs within the last 20 years have used Microsoft operating systems because I play a lot of games and that is the platform that supports all of them. All of my software positions have used a Windows OS or an embedded Windows OS so nearly all of my experience is based on Microsoft technologies.
At work I use Windows 7. At home my primary OS is KUbuntu (Ubuntu with KDE), I used Windows 7 for a long time at home (mostly because of nvidia optimus) but once I got a good working copy of linux running I got stuck with it again.
I used Linux for many, many years and hated Windows with a passion. Four years ago I was forced to use Windows for work, ended up not hating it, and eventually switched my home machine to Windows 7. I now look on most Linux fanatics as interesting and quaint people who, one day and with a lot of luck, might end up doing something that matters to somebody.
The work I do is computational linguistics. I used to use Windows and then switched to Linux (Ubuntu) on my laptop first, then on my desktop too, and I haven't looked back. My impression is that it has very clean memory management, and is very stable under full load and full memory usage. You can easily kill a process or two that is using most of your CPU / memory and restart it, and it is very snappy. One of the frustrations I had when using Windows (I had used Windows from 3.1 to Vista) was that I could not do this. In addition, for software development, I find little things like grep very handy. I used to game when I first joined this board as a teen, and I don't anymore so that's not an issue. Anyway, the point is that I am happy with linux and see no reason to go back. Try it out! Maybe you'll be happy with it too! :)
My notebook runs MacOS X, because it's a Mac and I can't be bothered to install something else and risk the hardware not being properly configured. I've grown lazy that way.
My desktop dual-boots Windows 7 (for games and Windows development) and Sabayon Linux (for POSIX development).
My gaming PC runs Windows 7, my netbook runs a linux flavor, my webserver in the closet was on some kind of outdated freebsd when I switched it on last time and my mobile is running iOS. Only God knows what my TV sets and DVD players run, that allows them to update themselves using my wlan router, running openwrt. My main system? I would be ........ed without any of them.
Linux is deliberately flexible. with that flexibility comes some difficulty, but the difficulty itself is not deliberate. Linux allows its users to modify parameters of its operation that windows doesn't even expose in a read-only context. if you want a user friendly solution in the box ready to go, sure, go ahead and use windows. on the other hand, if you have the knowledge and desire to customize your system at the lowest levels, then an open source operating system like Linux is the only choice.
I use windows mainly for gaming. But I recently installed Ubuntu as a dual boot.
At uni, we program on a unix system (I think it's red hat), so I thought I may as well program on linux.
Windows, because of graphics drivers and the need for true M$ Office. OpenOffice/LibreOffice is garbage. I am quite fond of Lubuntu though, very lightweight. Only a few extra packages to install and a few settings to change from a fresh install.
I take windows, with a healthy pinch of salt, for standard work, As was posted 9 out of 10 cats cant be wrong, but unfortunately my company has clients on unix services, and hosted variants too, and some of tasks, adding patches etc = mucho fear, it is just so unforgiving with the unix stuff, a wrong keystroke = ten client sites down..possibly, restore to previous nite backup, lose a days work for all the workers across all those clients, but thats not the OS's fault its the pony implementation. Heres a good 'nix nightmare... nearly nuked toy story 2
Windows 8 - because it rocks.
Prior to that several linux distros for as long as I can recall.
Windows 8 is broken because due to forcing UAC on you. It will remain broken until I have found a way to get rid of that problem.
so you're saying that in windows 8, there's no way to disable UAC? I guess I'll be sticking with 7 for a couple more years then.