Schoenberg presents a very thorough look at traditional harmony, and he is quite intentional in his approach-- his statements are justified by common sense, acoustics, and the successes of other masterful composers. Throughout, Schoenberg teaches the reader to be skeptical of "aesthetic laws" devised by theorists without reason. A careful read through Harmonielehre provides the following: 1. a rigorous look at harmony 2. a window into the mind of one of the 20th century's most influential composers 3. a snapshot of music-theoretical understanding at the turn of the century 4. an appreciation for an important text on music theory and aesthetics.
While the book is an impressive tour through harmony (from basic scales and triads through polychords, the octatonic scale, etc.), Schoenberg can be both exhaustive and exhausting. When he isn't providing the reader with a million examples of voice leading or modulation he takes up page after page with philosophical musing. There's even one passage (pg. 180) which requires a page-long footnote on Schoenberg's fascination with German etymology.
I appreciated the dual aspects of Schoenberg's musical thought-- his quirky understanding of harmony and his throughly Teutonic philosophy of music-- but I would have preferred that they be kept somewhat separate throughout the work. I understand why they must be presented side-by-side-- music theory has a history of dogmatism and quasi-political debate. Schoenberg couldn't have presented his view of harmony without some lengthy discussion and justification. As it is, Harmonielehre represents a dense and extended stream of consciousness.
This work really must be read over an extended period of time-- I found it quite difficult to read for long sittings-- and the reader should expect to skip over sections. Schoenberg supplies a plethora of opportunities for the student to practice, analyse, and contemplate; if all of these are taken, Harmonielehre truly becomes a year-long course in harmony. The student who truly wishes to learn harmony from this book should keep a piano close at hand. The student who wants to appreciate Schoenberg's text can do without.