I used vim for about 10 years before I finally gave up, and it was completion that did it for me. It was just so ludicrously hard to customize vim (writing vimscript - although the python integration wasn't much better) that it hurt. I was held back by not being able to customize it, and the fact that I couldn't get any better completion than ctags was insane.

My startup Circle is written in Clojure, so I was pretty much forced to learn Emacs, and the world is such a nicer place. I really wish I had learned Emacs 5 years ago - with the time I spent mastering and then trying to customize vim, I could have mastered and learned to properly customize emacs.