Seriously, how good is it? Pretty fucking good, I say.
There are so many good things about it, but I think the thing that just keeps me from never straying is the completion engine.
It all starts with C-n. And in fact, thats really all I use most of the time. And that baby can complete anything for me across a variety of languages. It completes me.
No other editors that I have come across come even close to VIMs completion, and it would be that hard. It boggles me why other editors can't seem to do it as well.
Forget function completion or variable completion or class completion or keyword completion. VIM completes everything. You've used a function that you haven't written yet, it will complete that, even when you are writing the definition for it. It will complete associative array keys, HTML element ids, files, tags on top of all the regular boring completions you see in other editors. It will aggressively search through your includes in an attempt to please you in some way. And it does it quickly too.
I just used it again. It completed an associative array key name via a couple of levels of includes. Sweet meats my friends, sweet meats.