OS X is prettier, less annoying, and generally just works. I'm not interested in system administration; I just want to get things done.<p>I do use Linux on servers and on my netbook.
Lacks Steam support.<p>Requires activities I am unwilling to engage in as a user of a system.<p>Less useful than competitors.<p>Uglier than competitors. Lack of uniformity in UIs. Large number of UIs that only a mother could love.
I used to use Ubuntu from 8.04 to 10.10 until a regular security update left the system in an unbootable state. Since I didn't feel like reinstalling my system from scratch I went back to Windows 7.
I have 2 computers/2 monitors side by side w/ one keyboard and mouse via synergy. 1 computer is linux, other is win7. Linux doesn't have creative suite, and I'd prefer not to use VMWare or screw with wine for something like that. And no, linux does not have real replacements for creative suite applications. I also do a bit of flash/flex/as3 development, not recommended on linux unless you have lots of free time on your hands and want to fight an uphill battle. There's a handful of other stuff that works better/faster on windows.<p>For the most part, I use each system equally, but would have difficulty choosing just one. My laptop is windows. Never tried apple much for varied reasons I won't go into.
PGP Whole Disk Encryption. Yeah, I hate my life too.<p>I do use Arch Linux at home for a server, heaps of Linux VMs for different things and almost all of our servers are Linux, a BSD or OSX.<p>But my main laptop runs Windows 7, and I'm typing this on a Mac Mini. The Mac Mini is just better plugged into the TV, Windows 7 is generally the best Windows implementation I've used and is good enough for everything I want/need to do.
I think the main reason is that there is so much going on in Ubuntu, or Linux in general, with every release. I really like to run the latest stuff, but 6-month cycle is not good for development machine, the updates sometimes break the system and I can't concentrate on my work, so the development system is a Mac, I have Ubuntu on other machines.
Because I use Chrome, Adobe Lightroom, Adobe Illustrator, Adobe Premiere and more and all of these run without the slightest problem in Windows 7, and all of my past Ubuntu experiences have ended with someone suggesting I edit ~conf.something and removing the number 7.
I like OpenBSD for my server needs and use OS X on the desktop. I might start using Dragonfly BSD for some server stuff since I am very curious about how the HAMMER file system works in practice.
The day that Adobe releases CS for Linux, is the day thousands of designers like me convert over. In the meantime, my system runs Windows.<p>My laptop however, runs Ubuntu.