Tony DaSilva has written a very excellent and thorough review of XMLMind XXE at http://www.writersua.com/articles/xmlmind/index.html. It's a very good editor, and something you should consider if you belong to an authoring group that is new to structured markup.
Tony's review is very informative. Even though I've used the tool before, I still learned something new: interactive margins!
I've used the Standard Edition to test DocBook 5 customization layers, and it works like a champ! It's one of the few XML authoring tools that supports RelaxNG validation, too. Just try finding that support in Arbortext or XMetal!
The editor also supports DITA, though I've found it's map support a bit weak. I'm sure that will change over time.