Recs.
Updated
SpecsUpdate
Pros
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 Self documenting
Emacs has extensive help support built-in as well as a tutorial accessed with C-h t.
Cons
Con Learning curve is long
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 Takes a lot of effort to configure to fit your needs
It definitely can be configured (and even programmed) according to anyone's habits, needs and preferences. For example, imagine a text editor which can handle typesetting and displaying English, Russian, Japanese and Sanskrit text, supporting LaTeX publishing workflow, Common Lisp development workflow, Javascript development workflow, writing down thoughts and to-do tasks and also having a full-featured command line.
But you will have to configure/program it, and it'll in no way be either fast or pleasurable.
Recommendations
Comments
Flagged Pros + Cons
Con Sometimes the extensibility can distract you from your actual work
Tweaking your .spacemacs config file can take half a day.