I love metal primarily for the music itself (rather than the lyrics, attitude, fan culture, etc.), so this book was refreshing in that it contains some valuable musicological studies of metal as a genre and of several specific songs. Walser discusses timbre, meter, modes, etc. and resonances with blues and European classical music (in particular Baroque music), using transcriptions of the music to make his case. He even suggests, quite intriguingly, that the fact that metal musicians are closer to Baroque composers like Bach than later composers such as Beethoven should tell us something about Bach and Beethoven, not just contemporary metal. A lot of the music theory was honestly over my head as a very amateur musician, but I understood enough of it to take away some valuable insights into the complexity of the music I love.
The cultural and historical parts of the book were interesting, but perhaps less novel. Be aware, you will need a basic proficiency in PoMo speak to understand parts of chapters two, four, and five, but it's not as bad as it could be considering Walser's use of social science and the humanities, where postmodern theory has, for better or for worse, become the basic assumption in recent decades (with the exception of philosophy). But the fancy PoMo stuff could be skipped (much like the fancy music theory parts) and you will still get a lot out of the book. The chapter on gender didn't break much ground (although the discussion of androgyny was interesting). The last chapter, "Can I Play with Madness?" had some great moments discussing Ozzy, Judas Priest and Iron Maiden.
Of course, this book is a bit dated (published in 1993), as other reviewers mentioned. If you didn't listen to metal in the 80's or early 90's, this book may make less sense, but I think the fact that bands like Poison, Bon Jovi and Van Halen hardly seem like metal to today's audience just underscores how mainstream metal became in the 80's and also how new most of today's sub-genres are. This isn't to say Walser's book is irrelevant. Fans of metal today can still learn a lot from this book, both about their roots and about the aspects of metal that are still present in today's sub-genres.