This is a great book for those who really want to "use" Visio to get under the skin of data they deal with. Many computer books tend to be long reference lists of properties and data, but this book's packed full of worked examples of real-world problems plus tips and code that you won't find in the Help menu. There's great coverage of the new 2007 functionality such as PivotDiagrams and Data Graphics, but all from the point of view of how you can actually use this stuff.
I got this book because I've seen David speak at various events and what you get here is his experience "in the field" as opposed to a simple tour of Visio's features.
If you're a complete beginner then there are probably more basic books available, but if you're already using Visio in your job then I'd definitely recommend it.
I found this book heavy going as it goes into depth very quickly without providing an overview of the basics.
There are chapters at the end that would provide a gentle 'lead in' if they were at the start.
In particular the VBA chapter should be one of the first and followed by a chapter discussing Visio's VBA functions and object syntax - the coding & coded cell examples provided throughout the book would probably make a lot more sense if this was done.
If you;re a very experienced (advanced?) Visio user this might make more sense.