A question that crops up fairly frequently on an internal list is the best way to convert docx to xps. Word is obviously a great tool for that (take a look at Document.ExportAsFixedFormat in the Word Object Model Reference), but it's not always suited, for example see KB 257757 Considerations for server-side Automation of Office.
The fact that, as in this instance, Word isn't always the best tool provides a nice illustration of the benefits of moving to open formats — if one vendor doesn't have the right tool for the job, the door's wide open for others to step in an provide a solution. In this case I noticed yesterday that Ecrion has added support for .docx files in their XF Rendering Server product (last time I looked, when they announced XPS support, they had WordML but not .docx/OOXML). Not only does this example show the benefits of interoperability, and of the widening ecosystem that's developing around XPS, but it's also a nice highlight that XPS isn't just Microsoft — if we have a solution great, if not there's an increasing chance that someone else does :-)