If you are willing to put in a little time to learn how GNU Emacs works, it can be an extremely powerful and versatile editing tool, as well as being used for almost any kind of text searching, replacing, or processing. Using Org-mode alone is worth learning Emacs. Once you learn the basics, they can be applied and used in all areas that you would use Emacs for. If you want a text editor and information tool that you can use for almost everything, then the learning curve is worth it. If you want simple text editor for editing small files, then Emacs may be overkill.
If you're doing any sort of text editing, you should consider Vim--once you climb the learning curve, you'll be faster and more efficient at everything you do. Also, Vim is my default search/replace tool--whenever I need to make changes throughout a document, most often, I'll copy and paste into a Vim window so that I can take advantage of regexp-based replacements. Vim isn't for the faint of heart, though--it's hard to learn and super complex. If you use a text editor once or twice a month, or just need a simple way to strip out formatting, Notepad will get you where you want to go without all the confusion.
Readily available: Emacs is available on almost all operating systems. You can use it for free, and even use it for remote editing. It has a nice desktop application that anyone can use, even the ordinary user.
Lightweight application: Although it hasn't always been, Emacs is now a very lightweight, efficient editor that isn't going to demand much CPU power. This can be crucial for large-scale applications.
Easy learning curve: for complex editors, Emacs is one of the easier to get a handle on. There's good documentation, and an easy layout that makes figuring out how it works much easier than others.
I don't consider the steep learning curve to be a hinderance on the overall usability. I would rate this a ten, but to be honest a lot of people do get hung up at the beginning and just abandon it. However, for people who have made the moderate effort to get over the hump, nothing can be more usable.
Since Emacs is free software, support comes from online forums, and online searches. I have always been able to find an answer to any question I've had, as well as "How do I do XXX in Emacs?". I've found Emacs users in general are very willing to help others who are learning Emacs and to share what they have already done with Emacs.
The support is somewhat lacking; Vim doesn't really have support outside of its forums. The forums, while notably good, aren't equal to real support though, and the steep learning curve makes you wish there was a help desk that would assist with all of the basic questions. There are several handy books and guides that can be found, and most implementations of the product come with "vitutor" which walks you through its usage.
Emacs can be easier to use than Vim as it is more similar to modern text editors. Its graphical interface is also better than anything Vim has to offer. Emacs also has better syntax highlighting and recognition of coding languages. Interestingly, Emacs handles clipboard copy and pasting into other programs a lot better than Vim does. Overall, if you know how to use Emacs, it is often preferred over Vim.
I think it comes down to usability (and frankly, just preference). There's an old adage that Emacs is a good Operating System with a poor Text Editor! Jokes aside, when looking at different text editors, such as Emacs or Nano, Vim is the one that is usually always preferred because of how simple it is to learn. The learning curve of Vim compared to other text editors just makes it such a natural choice.
Overall, the ROI of Emacs has stemmed around its nature as a free, open source product. Usability is high, so when you use it you are more productive, but if you are unfamiliar with it, you will be less productive to start.
Emacs is more than just an IDE. It includes IDE-like functionality, but it is really much more than an IDE.
Emacs makes it easy for me to work in my environment, in a fashion to which I have become accustomed over the last 30+ years. It is more suited to the professional who has used it before.
