I started using Emacs for a job that used a custom framework built around an old version of a rather non-mainstream language. I found some Emacs modes for the language, but they were a few versions ahead of what we were using. I was able to hack them pretty easily. The thing that amazed me was exactly how much else I managed to squeeze out of Emacs compared to the other editors I explored. I had editor commands to jump around and create new files and run and test our stuff. Yeah it's a long slow learning curve, but in the end I had built a decent development tool for this particular job.

If I had used any other editor, it would have just been a text editor, and nothing more. With Emacs I could build real tools and multiply my effectiveness. If this job weren't Windows-only (yeah...) I would have used TextMate, but I think I'm better for forcing myself to learn Emacs.