I've been using XPath and XSLT since 2000. XPath is a powerful and intuitive way to retrieve information from—or apply style to—your document. I've presented on it several times at conferences and in training sessions. XPath is very useful. Arbortext Styler's element rules are based on it but displayed in plain language rather than code strings by default.
Below are my XPath training slides. It includes the basics of writing XPath expressions, including examples of common use cases, as well as a picture for you to print out and hang on the wall above, behind, or next to your monitor. It can be extremely helpful in the early days when you're trying to figure out how to write the XPath expression.