I think it really depends on what you are doing. For instance, keeping in
mind that XML is a data oriented markup language, the tree structure view
provided by most specialised XML editors can be handy when using XML as an
alternative to a more traditional database. But, in my work I am dealing
with research documents using XML to structure the documents so as to make
data contained within the document more transparent while retaining the
traditional format of a research document. For this the tree-structure
method of presenting the document is not very handy. I choose to use
traditional text editors, the same ones I use for HTML editing, i.e. BBEdit
or Homesite, and adding my own XML code to the available configurable menus
so as to ensure validity. With this method I am always presented with the
document in its original format with the tags highlighted.
On the other hand, when I'm working with the document, ex., creating an XSL
document for presentation the tree-structure view is very handy.
My own personal opinion is that as yet (until the XML capabilities are
integrated into the software we use in our day to day work now, i.e., very
soon from what everyone is saying), one must choose one's tools depending
on what the task is. But, to sum things up, XML is simply a text based
document format, meaning that any tool that allows the export of documents
in a text based format will suffice.