Anybody who wants to work on a Unix or Unix-alike environment needs to learn vi/vim. It's not even slightly user-friendly:

• it has a bizarre modal structure;
• it has key bindings that are a bit of a stretch (h-j-k-l for left-down-up-right respectively, for example);
• its online help system assumes you know the odd terminology it uses and is thus doesn't help to learn the application;
• it doesn't play well with others in the GUI world (partially a result of the bizarre key bindings, but not entirely).

So why should you learn it? It's one of the only editors you're pretty much guaranteed will be on a Unix-alike. Any other editor (other than ed or ex -- don't ask!) is not guaranteed.

VIM is VIM, everywhere (I mean, it's available in the same way on all platforms)

The steeper the learning curve the higher the reward!!

Learning vi opens your world to all sorts of things like vimterpreter in your browser or even vim in your shell/command prompt.

Just 5 years using vim and I'm still optimising my and time/work ratio by simply reading more on vim.
Once you "get it" you'll think twice about "normal" editors and even fancy IDE's. (vim could be one if u want, just read)

did I mention the steep learning curve ;)

vim just woks. It is far nicer the emacs but then again the emacs/vim war has been going on for ages ultimately it comes down to witch suites you best for me it's vim for you it might be emacs.

I like it for a perfectly simple reason - I can use the same editor (with same settings) between UNIX and windows.

This was my favorite editor on Linux. It is seemingly infinitely customizable and extensible.