Just XML is not an easy book to grasp. Several of the other reviewers probably weren't up to the book. The book demands a lot from the reader.
You should be able to think in abstract terms before reding this book, or else it may well end up gathering dust on your book-shelf. You should have had exposure to XML before reading this book (in my opinion), and you should know a programming language. Any programming language will do, as long as you're familiar with concepts such as variables and functions, parameters and return values.
Just XML tries to be an "introduction" book, but I feel that some of the topics were just too advanced for an "intro" book. It dives into XML namespaces and XLink long before mentioning XSLT or DTD. The knowledgeable may wonder why; the answer to the question is probably that namespaces and XLink are quite useful, and have not received their fair share of exposure in courses, the press and so on. The author obviously feels that those technologies are an important part of XML, and so introduces them quite early. This is what might put the non-programmer off, because the XPath discussion gets quite technical before becoming practical.
All in all, I would say that the book is well worth it, if you're up to it.