Those who do not already love the musical may be frustrated with the writing. The story seems to jump around a bit. When Leroux is building suspense, he changes the subject, and comes back a chapter or so later. It's still a very interesting story. The bottom line is clearly expressed, just as it is in the musical So if you just go with the flow, you'll probably like it. The narrator does as good a job as I can expect with a difficult narration. Not outstanding, but fine.
These stories are part of the foundation of the mystery genre. These were one of the influences for Agatha Christie. The plots are clever and well developed.
The only thing I don't like is that the old man does not share his conclusions with anyone but Polly, who he relates his cases to. The criminals are not brought to justice.
The narrator does all the accents in some kind of Peppe Le Peu (the skunk). If you can get past that, the story is pretty good. Not nearly as good as Mystery of the Yellow Room, but worth reading. It too takes some overdramatic turns, but it eventually comes together. There are so few Gaston Leroux works that I recommend it.