There is something really sinister and dark about this book. It’s one of those where drug-taking/mental illness really pushes the plot, and it leads to some really gruesome moments. That atmosphere is the best thing about the book, I think: that sense that you don’t quite know what’s going to happen next, because it seems like it could be anything. It’s not very subtle, but it works pretty well.
Of course, I don’t love mysteries which rely on mental illness for their sense of danger and their motives, because the facts are that mentally ill people are more likely to be attacked than to be the attackers, by quite a margin. But it’s a classic trope and this is a classic book, from a less aware time, so while I wouldn’t recommend it if that kind of thing really gets to you, I don’t hold it against it too much in how I enjoyed it.
Mind you, considering I didn’t like the detective that much and found the Watson rather boring, I’m not sure how to rate it. The sense of atmosphere is definitely worth something, though, and it’s not as though I found it a hard read. Until I sat down to think about it, I’d have gone with three stars easily.