A very nice overview of what XQuery is, what are the relation to other XML based standards and why it is still very rare used is summarized here http://grtjn.blogspot.com/2011/10/xquery-novelties-revisited.html.
Once you want to dive into XQuery a bit deeper, get a feeling if the approach is sufficient for your use case you should try out BaseX. A XML database, open source. The main advantage for tryouts which are not available on alternative solution is a lightweight but very useful UI fronted for the content stored in the database.
Adding XML documents, try out queries and see how / what they match and last but not least a noticeable view of how information in a XML store looks like (from a conceptional point of view) comes aligned with easy install and ramp-up costs.
Alternatives (commercial and open source) are collected here: http://trent-intovalue.blogspot.com/2010/08/xquery-design-patterns.html