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Pros
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.
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.
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 Closed source
Mac OS is closed source itself, which means that it is developed more slowly and has more problems