TextMate seemed really awesome, as did BBEdit, so why not stick to what you know?
2) Speed - Closing out 1 year of working in a terminal more than a UI (from a decade prior of working with Windows), I am finding Vim more "convenient" (edit: another comment uses the term "efficient"). I am by no means an expert, but moving text around in Vim feels faster. It takes time to master, but I am finding value in the effort.
I still prefer GUI based editors for certain tasks - visual diffs for example (BBEdit, Cornerstone), and use a GUI for database development (DbVisualizer for schema browsing and simple ERDs), but plain old Vim is becoming a habit.
I think the problem with Textmate is that it appears to effectively be abandon-ware. I and many others are skeptical that a final, complete version of 2.0 will ever be released, and the source-code dump makes it seem even less likely. Second-system syndrome is alive and well.
In general, it's not a good idea to continue investing time and effort in software that will eventually cease to be available or updated. Given the persistence of vim and emacs, it seems unlikely that either will be abandoned in any reasonable short- to medium-term timeframe.
That's when I post links to articles like "Why, oh WHY, do those #?@! nutheads use vi?" http://www.viemu.com/a-why-vi-vim.html and "Coming Home to Vim" http://stevelosh.com/blog/2010/09/coming-home-to-vim/, "The vim learning curve is a myth" http://robots.thoughtbot.com/post/13164810557/the-vim-learni..., etc.
Even though I have several different editors on my Mac, including ones I've paid for like BBEdit, I've been pretty much using Vim exclusively for nearly 8-10 months and it's been great.
I started to get interested in Vim last year because of all of the blog posts from folks looking to switch from Textmate for a variety of reasons. And since Textmate was THE editor on the Mac for quite some time, I was curious why accomplished developers were moving away from it.
Of the many things I like about Vim, the one that sticks out for me: there's no ceiling on what it can do and what you can do with it compared to other editors. There's an internal consistency about it that I appreciate--the Vim "language".
Articles like this one "Learn to speak vim – verbs, nouns, and modifiers!" http://yanpritzker.com/2011/12/16/learn-to-speak-vim-verbs-n... really brought the concept home for me.
I hated DW's bloat and I knew how to write HTML/CSS/JS by hand so I turned to what was considered the top text editor on Mac OS 9 at the time. I liked it and used it for a few years until I upgraded to Mac OS X. At that time, I simply fell in love with the command line and the beauty of TextMate's snippets and Cmd+T and many refinements. The love story quickly turned into boredom and TM soon took the road to nowhere. This happened at a time I wanted to switch to Linux full-time so I spent a year trying every cross-platform editor/IDE I could find. I finally chose Vim which blew my mind and still does at time.
Curiosity paid, in my case.
I've stayed with Vim because I find it has very efficient means of manipulating text, and tends to stay out of my way. I feel encumbered when using Sublime Text 2, or TextMate (though less so with the former).
I do not understand the reason for this at all for people who don't know vim. There are plenty of other editors that are just as feature rich, if not more so (Sublime and PSPad come to mind). Also, if you need the GUI based macvim, you lose the most important feature of vim and the actual main reason for its popularity today: the ability to edit files locally or remotely inside a shell that supports only text.
Then again, choosing inferior technology is actually quite common these days, as the widespread use of Rails easily demonstrates.