Working for years with Oracle’s relational environment, XML DB is a completely different kind of beast. You’re confronted with all the languages and techniques the XML environment has to offer – XML, XML Schema, XSLT, Xpath, DTD, DOM, SAX, etc,etc,etc – plus the ones you know from Oracle’s (object) relational (database) world, SQL, PL/SQL, java, OCI, java, etc.
I am currently working for a customer, who implemented the XML DB functionality in a new system. The total amount of XML data is roundabout 200 Gb. The XML data is split-up in 4 tables, 2 tables containing an XMLType column (XML Schema based, CLOB storage) and 2 XMLType tables (XML Schema based, Object Relational storage).