Well, I did finish it. I did want to find out what happened, but I didn't enjoy this very much. Scored maybe 1 on the scary scale (out of 5). Because each chapter is about a new character reaching Slade House, I never got to meet any of them for long enough to care what happened to them. And because each of them end up in the attic where a particular scene is played out, we read that scene almost five times. One of those characters is an autistic boy, another a black woman, there's a fat teenage girl, a lesbian, a white middle-aged man - anyone would think David Mitchell was trying hard to be diverse.
The world Mitchell creates for the two main characters - twins - is too complex for so short a book, so that towards the end the twins are having conversations with each other about the rules of that world for the sake of making things clearer for the reader. But it comes out as a stream of operandi, glyphing, orisons, psychowaves, psychovoltage, lacuna, banjax... too much.