I'm not an Apple evangelist - I use a lot of different operating systems (FreeBSD, Arch Linux, Ubuntu, OS X, etc) at home and at work - I just happen to use OS X sometimes (and like it).
OS X is actually a pretty good operating system (the only major caveat to this being the filesystem, which is pants):
- The underlying OS is essentially a fork of BSD/NeXTStep - BSD is (very) well known for being stable, fast, and memory-efficient. OS X has absorbed many of these traits
- The last few versions of OS X are Unix certified - and because Darwin is based on BSD, it's also POSIX compliant... which means it's pretty much the same to use as on the command-line many other Unix systems
- The OS will run most software you throw at it - certainly most things that on a Linux machine with the help of
port. I've never had any significant problems, and I've been using my laptop for software development for several years. OS X is also very popular among lots of tech companies
- Hardware support is very good - printer/peripheral driver installation etc is plug-and-play in most cases, which isn't always the case with Linux or other Unix variants
- The desktop environment (Aqua) is excellent - it's good-looking, easy to use, and fast. The default multi-touch gesture control are pretty good (more usable than a lot of Linux distros and without all the faff of getting it set up), and the keyboard shortcuts for navigation are very good. Additionally, the accessibility tools on it are good - this might not be important to you, but it will be to some users
- It doesn't use GNU tooling, but you can install GNU tools if you like (through package managers like port, fink, brew, etc) so you can use all of your favourite command-line tools with minimal effort
- Apple do actually contribute to open source before you bash them for that as well (see CUPS, clang, etc - actually it's easiest if you just look for yourself)
- Macs aren't just overpriced Facebook machines - sure, the hardware's expensive, but that's to be expected - Macs are a marketed as a premium product. The hardware is usually pretty good, and the OS has the benefit of being developed specifically for that small range of hardware - which means that things like driver issues or kernel/graphics problems are less common.
- They're good for software development - they'll run any IDE you throw at them and the
xcodedevelopment tools are actually not too dreadful (although I don't use them myself)
- Last but not least... They're good enough for a fair few of the CS dept staff - 'nuff said
I'm not saying that OS X is the best for everything (for example, I wouldn't use it for a server OS - I'd probably use FreeBSD, CentOS or Ubuntu) - but every OS has a use case, and it just so happens that OS X is designed to be a desktop OS - and is very good at it.
So - why would you choose OS X? I dunno really, the same reason that you'd choose any easy-to-use, attractive, fast, stable, reliable, standards-compliant desktop operating system...