If there is one article you need to read in order to comprehend the magnitude of the semantic web, it is certainly this one.
From a user's point of view, XPath query strings are pretty darned geeky. I'm hopeless with them myself unless I have examples in front of me. I find that having a list of examples available in the context of my own live data, and synchronizing it to an input box in which examples can be modified, leads me to discover and record more useful patterns. A subtler thing happens too. As you're writing the XHTML, the search possibilities begin to guide your choices. [Full story at O'Reilly Network]