I go back and forth between OSes on a daily basis. My work computer is a Mac Pro, my home desktop is a self-built Win7 machine, my current laptop is a Lenovo w520 with Win7 and Ubuntu installed, my previous laptop was a Macbook Pro. While I'm fine using just about any OS, my order of preference for user interface is:
1. Mac OSX
2. Gnome 3 (it's a Linux desktop)
3. KDE 4 (another Linux desktop)
4. Windows 7
My workflow just feels much more effortless under OSX. It's hard to pin down why in tangible examples, but one does come to mind: the QuickLook feature. It lets you preview an item by pressing the spacebar. It doesn't require launching a separate program (like in Windows), can easily be made full screen, and you can move the the next image by just pressing an arrow key without switching back to the folder. Even if most of your workflow is in Bridge or Lightroom, I think you'll still find QuickLook speeds along everyday tasks--and after you get used to it, it just seems so obvious and simple that it feels like cutting off your fingers using an OS without it.
Finger gestures on a Mac trackpad are also so much smoother and easier than any Windows implementation I've seen. For that matter, two finger scrolling on Linux is a much, much better experience than in Windows, even when you're using the same hardware. I'm often exasperated by how slow and jerky scrolling is in Windows, and the more advanced finger-based shortcuts now used in OSX aren't even available on the other platforms.
As other's have commented, the headaches and lost productivity caused by viruses are pretty much eliminated by moving away from Windows. And while Win7 is much more virus and malware resistant than the nightmare years of XP, it still can't beat OSX or Linux. It's not just that those systems are less popular and therefore less often targeted. The Unix architecture (which OSX and Linux share at their hearts) is just a much more difficult environment for viruses.
I ended up choosing my Lenovo w520 over a new Macbook Pro purely based on hardware. I was tired of highly saturated areas of photos being dithered and blotchy as they are displayed on most laptop panels (including Macbook Pros, which are still only 6 bit displays, albeit good ones), and I wanted a screen that would show the entire RGB gamut. That narrowed down the list to only three laptops, the professional mobile workstations from HP, Dell and Lenovo. In the end, I got the Lenovo. It's got great pixel density (1080p on a 15.4 inch display), two internal hard drives (currently a 256gb SSD for Windows, Ubuntu and programs plus a 1TB drive for all my music, photos, videos and other stored data), a great Nvidia Quadro card, built to military ruggedness standards, and all the ports I could want (USB 3.0, ESATA, ExpressCard slot, etc). All that, and it's not even much heavier than my old Macbook Pro. If I could have gotten that hardware from Apple, I would have happily paid more for it just to get OSX. But I couldn't.
In a perfect world, I would be able to buy the hardware I want then put any OS on it I want. I have put OSX on a PC before, but it's not simple, and you have to worry about every update breaking the delicate network of hacks and workarounds you've made. As I said before, I prefer Linux interfaces to Windows too, but Photoshop and the rest of the Adobe Suite don't officially run under Linux, and even though you can get them working in WINE, there's lots of bugs. So, Linux isn't really an option as my production OS either.
I'm stuck with the OS that works best when paired with the hardware I want, which is Windows. I can live with that. I don't hate Windows, not at all. I just get a little frustrated when I know there are more refined, human-focused interfaces out there. And when Apple gets around to putting a screen with the same quality as an iPad into a laptop, I'll probably move back.