Ranked in these QuestionsQuestion Ranking
Pro Total customizability
Customizations can be made to a wide range of Emacs' functions through a Lisp dialect (Emacs Lisp). A robust list of existing Lisp extensions include the practical (git integration, syntax highlighting, etc) to the utilitarian (calculators, calendars) to the sublime (chess, Eliza).
Pro Keyboard-focused, mouse-free editing
Emacs can be controlled entirely with the keyboard.
Pro Works in terminal or as a GUI application
You can use Emacs' command line interface or graphical user interface.
Pro It's also an IDE
You can debug, compile, manage files, integrate with version control systems, etc. All through the various plugins that can be installed.
Pro Self documenting
Pro Great documentation
With 30+ years of use the Emacs documentation is very thorough. There are also a lot of tutorials and guides written by third parties.
Licensed under GNU GPL.
Pro Vi keybindings through Evil mode
Evil mode emulates vim behaviors within Emacs. It enables Vi users to move inside the Emacs universe.
Pro Mini buffer
You can pass complicated arguments in the mini buffer.
Pro Rectangular cut and paste
Emacs can select rectangularly.
Fully compliant GNU-emacs is available on many platforms, and they all understand .emacs configuration files.
Pro Visual selection and text objects with Evil
Evil is an extensible vi layer for Emacs. It provides Vim features like Visual selection and text objects.
Pro dabbrev-expand (Alt-/)
Dynamic word completion.
Pro Lisp customizations
Pro GTK+ widgets support
Pro Helm plugin adds even more power to Emacs
Con Learning curve is steep
While it's better than it used to be, with most functions being possible through the menu, Emacs is still quite a bit different from your standard editor. You'll need to learn new keyboard shortcuts.
Con Chorded keyboard combinations can be baffling
For example, for navigation it uses the b, n, p, l keys. Which for some people may seem strange.
Con Default keybindings may cause Repetitive Strain Injury
Richard Stallman himself, the primary maintainer of GNU Emacs, suffered crippling RSI for years. Unlike editors that allow a relaxed, home-row oriented approach to using the keyboard with only occasional need to use a modifier key, Emacs requires frequently contorting hands to use several keys at once, giving rise to jokes about EMACS being a euphemism for "Esc-Meta-Alt-Ctrl-Shift".