I mostly loved this book. Lovely descriptions, great characters, poignant moments, and set in and around the music industry just before the time of rock megastars...so that some of the real people who later become megastars (Syd Barrett, David Bowie, even the Beatles) drift in and out of the scenes.
I'm not quite old enough to remember this period, but the tail end of it was visible to me as I grew up. Denmark street was still full of decrepit music shops, and I remember some of the clubs that he writes about - especially Bunjies, where I often went with friends on a Saturday evening.
I even quite liked the way that characters and places from David Mitchell's other books turn up for a while, and then slide out of the story. Some of it is set around Gravesend, also an important location in The Bone Clocks.
But (spoiler alert) there's a part where Mitchell's supernatural frame-tale of a conflict down through the ages between evil drinkers of a human souls and their eternal opponents organised in "Horology" becomes important to the plot, and I really didn't like that. It must be really important to Mitchell, but it feels to me like a turd on an otherwise beautiful carpet. It could so easily have been done without, which only makes it even clearer that he really cares about this.
It did spoil my enjoyment a bit, but on balance I still really liked the book.