I'd say that XSL is handy for formatting XML data into something human readable, so you might want to learn that.
XMLStarlet is indispensable for testing against DTDs and testing XSL transformations.
Other than that, just remember to look at XML for what it is good at rather than a one-fits-all solution. There was some hype around it that seems to have died down...remember, it's just a tool.