It’s *nix and THE most advanced one
Programmers need to use a lot of command line tools on daily basis like ssh, curl, build tools etc. There’s nothing like a bash shell in *nix systems(Unix, Linux. OSX is Unix-based), basically you’re left to choose between OSX and thousands of distros of Linux. (You may use CYGWin in Windows but i think it’s terrible) In my opinion, most Linux distros are still not very user-friendly (Ubuntu is probably the best you can find), OSX provides a pleasant-to-use GUI while preserving the greatness of the command-line environment. Out of the box it has everything a modern OS should have, this saves you the time needed to configure the development environment and focus on the job you need to do.
OSX is beautifully designed, slick and always a joy to use. Helps you stay in a happy mood and be productive! Once you get used to it, you can never go back. (Actually not exactly, I still use Windows and Linux a lot )
Macs are great computers
One of the things Apple did great is the top-notch hardware design and seamlessly integrate with the software (OSX). The sharp and color-accurate displays, comfortable keyboard, back-lid keys, high performance touchpad with cool gestures, long battery life and the precisely-crafted aluminum unibody. All these contribute to a solid computers that developers can rely on.
OSX is inspiring
The OSX GUI is beautifully designed, pay lots of attention to details and emphasize on usability. As a developer, you can learn a great deal from it, just by following the design guidelines you can build a pretty decent app. It is a great example of how to hide the complexity from users (Installing apps with Drag-N-Drop, handy gestures, Seamless software updates), while still keep the “power users” happy (with advanced features like the Terminal, Automator etc.)
OSX is productive
Couple of features makes OSX much more productive than Windows and Linux. Here are some of my Favourites
Let you easily put commonly used Apps and files in the dock bar, and access it anytime. (Windows 7 has the dock and familiar desktop interface too, which makes it also a decent OS, but Windows 8…Microsoft what were you thinking?)
Multi-desktop and Mission Control
Separate different tasks in different desktops let you focus on single task without lots of clutter. Mission Control lets you have a bird-eye-view of all the apps opened.
Hitting spacebar to preview the file? that’s the best features ever added to an OS! Imagine you just want to quickly look what’s inside many psd files which are 100MB each, doing this on windows with Photoshop… (well you can install ACDsee, but it’s not free and still slower than Quicklook)
Some task is just way more efficient to be done with the command line (batch jobs, tasks with simple parameters input), with the shell deeply integrated with the OS X, you can work with GUI and the terminal seamlessly.
I believed many developers use OSX for the same reasons I mentioned above (Or they just have to use it because they’re Apple developers). I grow up using Windows, but OSX is now my “daily driver”, I can’t write a program efficiently without using it (well…at least give me a Linux). I really hope OS X won’t change to touch-optimized UI like the Windows 8. Keeping iOS and OS X separate is the right choice.