It was interesting and had a good amount of depth for the different issues they've seen. The only problems I have with the book are with the actual file system and implementation - the fixed-size cache, the weird indexes (why would you have an index by size? there's no mention of its usability anywhere), and, of course, the horrifying C++ API, which tries so hard to use inheritance and multiple inheritance that's just painful to watch.
There's, of course, the benefit of hindsight. In the last 20 years, the filesystems have become a lot better (to compare, this one was written when ext2 was the filesystem for Linux, and it can't hold a candle to ext4 that's in use now).