I used it a while back in a previous incarnation of the editor. It is a "Outline Editor" for use in what is called "Literate Programming". My view of it was that you write documentation and embed code in the documentation and then you can use a mechanism the editor provides to output just the code (when you want to compile it or execute it). The editor made it easy to hierarchically organize and rearrange code the way you wanted it and had a way to output just the code for use in compilation etc.

The current iteration seems to have added more features while basically being a outline editor -- imagine keeping code in Emacs's Org mode and being able to compile it.

I liked it, but it felt like it didn't have all the bells-and-whistles of modern IDEs and required a different way of dealing with code.