Recs.
Updated
SpecsUpdate
Pros
Pro Easy to build graphical applications
Xcode makes it very easy to develop graphical applications (Visual Studio on Windows is also very good). Libraries and tools are included and standardized. If you want to build a cross-platform graphical application though, OS X is the limiting factor, with the fewest commonly available widget toolkits. See here for a list. Other than Cocoa, the native OS X interface, if you develop on OS X with any of those widget toolkits, your program should be portable across Linux and Windows.
Pro Good package manager
Homebrew has become the defacto unofficial repo. For more information about this see slant page on Homebrew.
Pro Easy access to lots of great dev tools
There's a large selection of great development tools available for OSX. The operating system itself comes bundled with a powerful terminal emulator, called Terminal. Additionally, Apple provides tools, like Xcode, an IDE that contains a comprehensive collection of tools for developing OSX and iOS software, for free.
Pro Streamlined workflow between devices
Because this is an Apple product, there is a streamlined workflow between your computer and all mobile devices. For example, if you type an a Pages document, once you save, you can open the updated document just moments later on your iPad, and vice versa. The same goes for iMessage, (yes, you can text people with your phone number from your computer. Actually, you can text other people with apple devices with just your Apple ID, with or without a phone number, for free!) Numbers, Notes, Reminders, Contacts, and just about any other Apple workflow application.
Cons
Con Most software is closed source
For people who like to use open source tools for their development work, this may be a problem. There's plenty of advantages to open source software, one of which is the ability to tinker with and customize the tools themselves that you are using. Although there's plenty of FOSS tools available for Mac, especially through Homebrew, the number of packages available is much lower than the number of packages available for any Linux distribution.
Con Can't replace the window manager
You can't install a better shell, like GNOME, KDE, or i3. You're stuck with the default WM - the most you can do is run an automation layer on top of the existing shell to handle resizing/moving windows.
Recommendations
Comments
Flagged Pros + Cons
Con Closed source
Mac OS is closed source itself, which means that it is developed more slowly and has more problems.