As anyone serious experience with converting (complex) XML documents using Docbook DTD with PrinceXML?
I am aware that PrinceXML can deal with such documents by throwing in a bunch of CSS...not sure if this is the way to ago.

One method is to use the DocBook XSLT system to generate HTML, then style that with CSS. As that takes advantage of the existing processing steps it might actually be easier than starting from scratch with your own CSS.