Without a doubt the most challenging book by Mann I've read. It was a rewarding read, however. The musical theory part of it is demanding, and I would be lying if I claimed to have understood it all, but I think I got the important parts.
It was written during Mann's exile, and it really feels like a book trying to gather of "German-ness" what's left worth saving.

I feel like trying to write more here would end up becoming an essay, and I am sure other people have said the things I want to say already anyway.

If you've never read Thomas Mann, don't start with this one. If you've read Thomas Mann before and aren't sure whether this is worth it: read it. It is.