Recs.
Updated
Vim is a powerful, portable, keyboard based text editor. Being text-based, vim is lightning fast, with an incredible set of features developed over its multi decade existence.
SpecsUpdate
Pros
Pro Everything is mnemonic
No need to memorize different key combinations for things like deleting the text inside of a block or deleting the text inside of a pair of quotes. It's just a series of actions, or nouns and verbs, or however you prefer to think about it. If you want to delete, you select "d"; if you want it to happen inside something, you select "i"; and if you want the surrounding double-quotes, just select ". But if you were changing the text, or copying it, or anything else, you'd still use the same "i" and ". This makes it very easy to remember a large number of different extremely useful commands, without the effort it takes to remember all of the Emacs "magic incantations", for example.
Pro Has been supported for a long time and will be supported for many years to come
Pro Asynchronous I/O support
Since Vim 8, Vim can exchange characters with background processes asynchronously. This avoids the problem of the text editor getting stuck when a plugin that had to communicate with a server was running. Now plugins can send and receive data from external scripts without forcing Vim to freeze.
Pro Has multiple distinct editing modes
Interaction with Vim is centered around several "modes", where purpose and keybindings differ in each.
Insert mode is for entering text. This mode most resembles traditional text entry in most editors.
Normal mode (the default) is entered by hitting ESC and converts all keybindings to center around movement within the file, search, pane selection, etc.
Command mode is entered by hitting ":" in Normal mode and allows you to execute Vim commands and scripts similar in fashion to a shell.
Visual mode is for selecting lines, blocks, and characters of code.
Those are the major modes, and several more exist depending on what one defines as a "mode" in Vim.
Pro Can never outgrow it
The fact that very few, if any, people claim to be a "Vim Master" is a testament to the breadth and depth of Vim. There is always something new to learn - a new, perhaps more efficient, way to use it. This prevents Vim from ever feeling stale. It's always fresh.
Pro Keyboard-based, mouse-free interface, and trackpad support
There's no need to reach for the mouse or the Ctrl/Alt buttons again. Everything is a mere key press or two away with almost 200 functions specifically for text editing. Vim does support the mouse, but it's designed so you don't have to use it for greater efficiency.
Versions of Vim, like gVim or MacVim, still allow you to use the mouse and familiar platform shortcuts. That can help ease the learning curve and you'll probably find you won't want to (or need to) use the mouse after a while.
Pro Extremely portable
Vi/vim exists on almost all Unix-like platforms. It's the de-facto Unix editor and is easily installed on Windows. All you need to make it work is a text-based connection, so it works well for remote machines with slow connections, or when you're too lazy to set up a VNC/Remote Desktop connection.
Pro Lightweight and fast
When compared to modern graphical editors like Atom and Brackets (which have underlying HTML5 engines, browsers, Node, etc.), Vim uses a sliver of the system's memory and it loads instantly, all the while delivering the same features. Vim is also faster than Emacs.