I'm using OS X for all my desktop needs, which are coding, network- and sysadmin things, along with the occasional writing and whatever else a single-person developer shop needs to do. Here's why:

1. OS X is the clear choice for a desktop system that just works and has all the power of Unix underneath. I've used various Unix desktops in the past and they all had one thing in common: You had to spend a lot of time to get things working to some kind of acceptable level, you had to spend more time keeping them that way and in the end you often get "you cannot do that" or "Software X isn't available, you have to use Wine/a VM/an inferior clone". I also use Skype, despite it's security issues and uglified Mac client, because it's the easiest to explain to clients[1]. I haven't tried using Skype on Linux myself, but I've heard things like "Sure, lemme just swap audio drivers and reboot real quick". Err, really?

2. Applications. On OS X you get some standard things that you may need, like Photoshop and Skype and a mail client along with a great choice of text editors. You also get Pages and Keynote[2], both of which are fantastic when you need to write documentation or a presentation and have them look effortlessly pretty and export to all kinds of file formats. On top of that, there are some pretty awesome apps from small and mid-size developers than you can get only on OS X. First and foremost of these is OmniGraffle, which has not only saved me tons of time but earnt me several "wow, I didn't know Visio can do that" comments (It can't). Oh, and if you can't avoid dealing with BigCorp clients, you can get Microsoft Office for those pesky .doc files they insist on sending you that always look a bit wonky in LubricateOffice.

3. You still have X11 for Wireshark (that's all I ever use it for anymore).

4. Time machine is a convenient zero-setup-required backup that will save you time when you screwed up and need that file back. It's seriously fun to restore a file with it compared to any other backup software I've ever used ("please insert tape labeled .. into slot 7").

5. Same for FileVault (full disk encryption).

6. I don't know what you do, but if you ever decide to pick up OS X or IOS development, you already have everything you need.

7. The font rendering is oh so very pretty.

8. If you happen to have any iDevices, they'll shut up and sync.

9. The hardware looks nice, is well designed (in practical terms) and when it breaks you get pretty good service. And Retina is awesome.

I guess it all boils down to how much time you want to spend fucking around with your system instead of getting work done. Note that you still can fuck around with OS X to a great extent (GeekTool), it's just you don't have to. Same reason I use FreeBSD (instead of OS X or Linux) for servers and Windows for gaming. You can certainly get some Windows games to run in Wine or learn how to navigate GNU info to read docs, but that's time and frustration I'd rather not inflict on myself.

[1] "Just give me your Skype username. Don't have one? Click that." is a lot less trouble than "Install this SIP client and then open a port in your firewall and follow this 35 page instruction manual". I wish it wasn't so, but it is.

[2] iWork costs a few extra coins but is well worth it.