Tools are a personal choice. Authoring using structured data, you can edit SGML/XML in something as simple as using Notepad (not recommended) or inside large and complex Content Management Systems (CMS). The choice is usually cost which is contrasted to quality. Typically, the more expensive a tool the higher the quality of the output or its content. For example, managing 15,000 XML data modules from the S1000D specification using notepad, would be out of control, but managing those modules under CMS is doable and enjoyable. Quality would rise, because the tool would allow more reuse of items and tracking all of the associate items, such as Illustrations and entities.
My experience with structured tools started with Arbortext Editor now with PTC, Oxygen XML Editor, and some work with XMLspy. Mostly I use Arbortext Editor, it has been a tool that my employer purchased. We have been using it for over 10 years, (even on Unix). I have used Adobe’s Framemaker, but found that it could not do Mil-Spec like work, so I drop it. If you get to define your own output, it is a wonderful tool.
What tools are you using?
Douglas