Briefly, until I have time to write something more, the import function will read the entire document structure in the /assets/import/ directory, including subfolders, and will recreate that tree (using your default settings for new documents, which you can set in Site Administration) in MODx, importing any .htm, .html, or .xml documents. You can control the parent document in MODx to import the structure into it as well. If it finds a title tag in the document, it uses that as the page title, otherwise, the filename is used. Otherwise, it imports whatever is between the body tags only, into the content for each document it creates.
So I typically take a static site, strip out the general layout of the pages into a template (or copy Dreamweaver templates) marking appropriate areas for editable content with the MODx [*content*] tags, and then clean up all the pages in my favorite editor, leaving only what I want to appear in the content of the document between the body tags. Not completely automatic, but very handy for large sites being converted to MODx. If you do it in stages though, you can easily just import the site into blank templates at first while you create a fancy new XHTML/CSS-compliant one (come on, you know you want to

)
The script is very small and easily customizable (located at /manager/actions/static/import_site.static.action.php). In fact, Ryan and I used a custom version of it I wrote for a project a few months ago, to import 100’s of Word documents we turned into HTML using an army of tools and people (
ok, maybe I exaggerate now and and again, but it is amazing the hoops you have to go through to turn a simple, cleanly formatted Word document into simple, cleanly formatted HTML 
) into a custom data base.
Pretty handy little script actually. Let me know if you have specific questions until I create a formal document, or someone volunteers to do it for me... (sigh)