Vim(Vi Improved) is a free, open source text editor, created by Bram Moolenaar and publicly released in 1991. Vim is one of the most versatile Text Editors and one of the best substitutes for an IDE (Integrated Development Environment) given the amount of plugins available, that make Vim almost an IDE.
Vim is available for Ms-Windows, Unix, and Mac. Independent ports are available for mobile, both for Android (Vim Touch) and iOS.
Pro Extremely portable
Vi/vim exists on almost all Unix-like platforms, it is 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 Can be extended to have virtually any feature other Editors/IDEs have
Vimscript provides a rich scripting functionality to build upon the core of vim. When combined with things like Tim Pope's Pathogen plugin management system, it becomes easy to add support for syntax, debugging, build systems, git, and more. Vim can be made into a sophisticated IDE with just a few plugins, such as YouCompleteMe and CtrlP.
Pro Saves times by avoiding the mouse and using keyboard-based commands instead
You never have to reach for the mouse or the ctrl/alt buttons again. Everything is 1 or 2 key presses away with almost 200 functions specifically for text editing.
If you insist on a GUI, versions of vim like gVim or MacVim allow you to use the mouse and familiar platform shortcuts, which can help ease the learning curve.
Pro Excellent performance
Because it loads the whole file into RAM, replacing all string occurences in 100MB+ files is quick and easy. Every other editor sort of died during that. It is extremely fast even for cold start. Vim is light-weighted and very compact. In terminal, it uses only small amount of memory. Anytime you invoke vim, it's extremely fast. It is immediate, you can't even notice any time lag.
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.
Pro Has multiple distinct editing modes which facilitate the workflow
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 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 No need to memorize key combinations since everything is a 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. You want to delete, so you select "d", you want it to happen inside something, so "i", and you want the surrounding double-quotes, so just ". 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 number of different extremely useful commands, without the effort it takes to remember all of the Emacs "magic incantations", for example.
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 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 Useful undo features
Vim does not only offer unlimited undo levels, later releases support an undo tree. It eventually gives the editor VCS-like features. You can undo the current file to any point in the past, even if a change was already undone again. Another neat feature is persistent undo, which enables to undo changes after the file was closed and reopened again.
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.
Con Requires Brain Mode Switching
When editing in vim, you have you use the vim keys; when editing in every other window on your PC, or in Word or Excel or other application, you need to use the standard system key combinations. Learning the vim combinations can actually make you SLOWER at everything else.
Con Poor support for external tooling
Many plugins depend on optional Python and Lua features, which may or may not be included in whatever binaries are available for your system. And without platform-specific hacks, it is difficult for plugins to operate in the background or use external tooling.