This book is very well produced and executed. Heavy use of graphics, clear text, excellent glosses on the side of each page to highlight important reading. The graphics are generally lovely examples of how to optimise your graphic design so as to "channel" the reader toward the take-home messages of quantitative data.
These examples intersperse the text very neatly. Indeed, the layout of the book itself is meant to be a talking point, and is certainly very rational and a pleasure to flick through.
However, I find the examples given can be repetitive and the text can lack depth without a sense of depth to the analysis. I would have preferred something a little academic in the reading. As it is, it reads quite like an online tutorial (which are freely, widely available...). In fact, I think you could perhaps garner more from a well-chosen blog or two.
Much of the "exemplar designs" selected by Tufte are also repetitive, especially between his books. While the examples in question are undeniably outstanding (e.g. particularly elegant train schedules and Minaud's military escapades from Paris to Moscow), I find myself feeling, "I get the point! Show me something new!"
Lovely as a reference book, but if you've already done extensive reading on this sort of thing (on blogs or otherwise), you're probably looking for more depth -- so maybe try elsewhere. If you're looking for a reference book that will consolidate the basics in a beautiful way, then this is an excellent, appealing solution!
(I am very happy with my purchase!)