Prior to four years ago, my work machine always ran Windows. At home, I usually had a desktop running Ubuntu Linux, but the laptop was Windows XP for a solid decade.
Then, I was issued a MacBookPro, and quickly learned to enjoy OS X. What I liked best about the MacBookPro wasn't the software, but the hardware. It was incredibly stable and almost never crashed.
I speak about the MacBookPro in the past tense because it was recently replaced by a Dell laptop running Windows 7. I've been working exclusively in Windows for the past 3 weeks and I must admit I prefer the Windows OS to OS X. I've just had to remember to CTRL instead of CMD.
Windows gets a bad rap, but it ain't so bad. Of course, I haven't tried Windows 8 yet... and I don't ever plan to.
What OS do you prefer on your desktop or laptop?