Walter Frisch writes in the new foreword to Schoenberg's Theory of Harmony that it "should be required reading for everyone interested in Schoenberg, in the history of harmonic theory and practice, or in the Austro-German culture of the first decades of the twentieth century". It is hard to disagree with such a statement, but it also delineates clearly who such a book should be relevant for.
I have to agree with the many writers who have already pointed out that this book, while not entirely unsuitable as a harmony textbook even today, is eclipsed by newer textbooks. I used Piston's Harmony book for learning purposes, but even that one is to be considered almost as old as Schoenberg's book these days; I don't know how the newer books are, but judging from the number of releases during the last decades there has probably been quite a bit of innovation even in such a field. The harmonic language described may not have changed much, but pedagogy definitely have. I therefore cannot wholeheartedly recommend this as a theory book for novices.
What is left, then, is an enormous amount of philosophising about the nature of the rules of tonality, and how they came to rise. Schoenberg himself says that he is a composer, not a scholar, and many of his ideas do, indeed, seem to be taken out of thin air. His thoughts on parallelisms seems particularly pitiable today, but historical benevolence must be granted: In the hundred years since the release of this book an enormous amount of information regarding medieval and renaissance composition and performance practice has been uncovered, things Schoenberg could not have known. For a mere mortal it is of course consoling to see that even a giant like Schoenberg could be so wrong when he acted on his intuition. The book is therefore somewhat flawed when it comes to explaining the philosophy behind common practice harmony, which is the field where this book seems to get the most respect.
In the end, the book's value comes from the fact that it is written by one of the pivotal composers in music history, and gives us a window into his thoughts. I realize that many reviewers state that the book had ignited their interest in harmony, or even composition, an interest previously dulled by more succinct and to-the-point textbooks. I can definitely sympathise with this view, as the book gives a lot more food for thought than your average harmony book does. On the other hand, it has always been my opinion that people who are easily bored by textbooks are so because they are unable to think of it's larger implications and possibilities. Unused is probably a better word that unable, because it certainly is possible to learn. And while one should not scoff at this book in that respect, there are many roads to Rome, after all, it is in a way dangerous due to the fact that Schoenberg's opinions may be taken as fact. Of course, it is a good and thought provoking read, but it must be read as a hundred year old book, that is, with the benefit of hindsight.
In that respect I would have wanted (and sceptically hoped for) a new edition where we were explained, in linear notes, where Schoenberg goes wrong in his musings on the history of theory, and, to the degree it is possible, which contemporary sources (in the broadest sense of the term) made him do so. Alas, such a thing was not to come for the 100th anniversary, and one is therefore left wondering whether it will ever come. Frisch's foreword is interesting, but not exciting enough to warrant buying the new edition, and I can't find any noticeable differences in content between this edition and the earlier one from my university library, save the introduction by Frisch. So while the book is primarily for those interested in Schoenberg it could have been made more useful by a thorough questioning of Schoenberg's claims, but unfortunately it doesn't look like anything like it is in the horizon.
I feel a bit bad about giving three stars, especially since the new binding, though paperback, is very nice, and the print is as good as anything out there. But the bottom line is that this book is useful, but for fewer people than one might presume. For the present day reader I find it first and foremost about Schoenberg, like Frisch.