Recs.
Updated
SpecsUpdate
Pros
Pro Self documenting
Emacs has extensive built-in help support, as well as a tutorial accessed with C-h t.
Pro Enormous range of functionalities (way beyond simple "text editing")
Emacs can be programmed to do a variety of things including multimedia playback (EMMS), typesetting (with the LaTeX language and AUCTeX), viewing images, working in the filesystem (dired), running a terminal emulator (ansi-term), IRC, email and newsgroups (Gnus), and much, MUCH more. Because Emacs is so portable, you can take you environment with you wherever you go, no matter the OS, and work the same way regardless.
Pro Backed by Lisp
Lisp is one of the most popular programming languages to date and is highly practical for carrying out software/application development.
Pro Total customizability
Customizations can be made to a wide range of Emacs' functions through a Lisp dialect. A robust list of existing Lisp extensions include those that are practical (git integration, syntax highlighting, etc) to the utilitarian (calculators, calendars) to the more sublime (chess, Eliza).
Cons
Con The least intuitive UI ever designed for use by humans
The Emacs interface predates the mainstream adoption of the graphical user interface by more than a decade and it shows terribly in the unusual conventions Emacs adopts for navigating buffers, manipulating the kill ring, undo and redo, and other behaviors. For those of us who grew up on Windows and/or Macintosh, the differences can be jarring and that only serves to lengthen the already quite lengthy learning curve. Emacs is profoundly unintuitive and I find myself shaping my thoughts to be more in line with Emacs, not the other way around.
(Protip for newbies: Meta (M-whatever) is really Alt on your PC keyboard, and Option on your Mac. (Mac users can set it to be the Command key instead.))