I currently use Windows 7 x64 Ultimate on my Lenovo G555, and I use Fedora 18 x86 on my MSI Wind U100. I have had lots of computers with lots of differet OS' over the years, so I'll just focus on the most recent ones.
I have been using Windows 7 since around 2010, and it has served me very well for the most part. I was using Windows XP Pro MCE before that, because I had decided I would never install Vista on one of my own machines.
Fedora 18 is new, so obviously I haven't been using that long, so far it is stable and performs better than Windows did on my netbook. I was using Fedora 17 for a while before this, and that was without a doubt the best free distribution of Linux I have used, so I figured may as well try 18.
I don't like Windows 8 so I probably won't be using that. I used to use Kubuntu, no spelling mistake, but I just like Fedora much more now due to performance mainly. Windows 8 just doesn't seem right for a desktop computer operating system, in my eyes.
I would probably be using Fedora 18 x64 on my laptop, if not for the fact that most of my games are only compatible with Windows. Since aside from that there is nothing holding me back from doing so. I am glad that Valve are making efforts and encouraging other developers to get more games on Linux operating systems.
Reasons I like Windows 7; good performer, customizable, easier for the most part, great hardware and software compatibility, does what I need it to do.
Reasons I like Fedora 17/18; great performer and doesn't guzzle RAM like Windows, completely free - to own and modify, customizable, does most of what I need it to do, easier than some other distributions of Linux.
My suggestion to anyone that is using a Mac, is to give some Linux distributions a go, you might discover you prefer them.