XQuery and XSLT

Hi list

As a fan of XSLT and almost addicted to it, I am surprised to learn that
XQuery is not just for views joining documents, filtering, grouping and
sorting, but that XQuery is also great for transformations like XML to HTML
and XML to XSL-FO/pdf.

I am wondering: What are the strong and weak sides of XQuery and XSLT making
it necessary to learn both?

Cheers,

Jesper Tverskov
www.xmltraining.biz