It's my default text editor now, for any language. (Vim is my backup editor for situations like sshing into a system that doesn't have Textadept installed since Vim is usually installed and if not that, at least vi is.)

My main reason for switching over to it finally was the ease in which new languages can be added to its support. I program in a lot of ... uncommon languages. Languages which lack representation in lexical coloration in editors (SNOBOL4, say, or Rexx, or Logtalk) or which, when "supported" are done so lousily it may be better not to use it at all. With Textadept I can whip up basic lexical coloration without breaking a sweat (usually under an hour barring weirdness like SNOBOL4's) and can do fancy tricks like having a separate lexer for each dialect of a language (SWI-Prolog vs. GNU Prolog, for example) without a whole lot more work. Further I can trivially handle embedded languages (like SQL embedded in C or separate the coloration of the Java and HTML components in a JSP page) and, even more fun, I can inherit lexers.

For an example of that last item, my Logtalk lexer is basically all of my Prolog lexers (complete with the dialect switches) with the Logtalk extras layered on top, so although Logtalk is a more complicated language than Prolog, lexically speaking (it's all of Prolog plus more), my Logtalk lexer is about 20% of the size of my Prolog lexer. And if I fix a mistake in my Prolog lexer, or enhance it in some way, my Logtalk lexer comes along for the ride.

I've not seen any text editor that comes close to that degree of lexical coloration support, and its support for snippets, tagging, folding, etc. is also nigh-on best of breed.