What are the advantages of Emacs for Mac OS X users?
You can use a OSX GUI Emacs in the form of Aquamacs which is configured to be more OSX like it its key commands and menus and integrates with the OS.

One appeal of emacs is because emacs is mainly written in the language (e-lisp) that users can use to extend it there are a lot of scripts that add all sorts of functionality to the editor making it nearly as powerful as an ide. Even configuration is written in the same language so once you learn it you can modify anything you want.

The other reason I use emacs is that it runs on several OSs (Windows, Linux, Solaris and OSX ) and so I don't need a different editor for each one as I would if I used BBEdit etc.