Vim and Emacs weren't enough?
Having an editor that I could extend in a real programming language (and JS is actually very nice once you grasp stuff like prototype inheritance and learn to avoid the... uhm, bad parts) would be simply awesome. And if it was pretty and integrated with OS X I would be willing to pay hard cash for it.
With today's web frameworks, when I hop between files all the time, I want a neat visual file tree.
I find it much more useful than the file trees in any of the gui editors I've tried. You don't need to leave your keyboard and you can create, delete and move files directly from the tree which some gui editors don't even allow you to do. If you haven't tried it give it a shot.