Don Box loves XPathNavigator because it is based on XPath (http://www.w3.org/TR/xpath) instead of an XML data model.

Don very articulately and humanely explores XPathNavigator in "Passing XML Data Inside the CLR" on MSDN TV at:

http://msdn.microsoft.com/msdntv/episode.aspx
    ?xml=episodes/en/20030603XMLDB/manifest.xml

He goes on to explain that instances of XPathNavigator comes from XPathDocument. This is important to me because XPathDocument objects can be passed to Transform() method of an instance of XslTransform.