Not successful
Many annoying things about this story - not least the spurious conversations with (now) dead musicians from Sandy Denny to Jerry Garcia - presumably selected because the dead can't sue. These are pretty unconvincing and sometimes cringeworthy as they are showhorned into the narritive.
The narrative arc of the band's rise to fame is totally formulaic with the exception that the relationships within the group have no tension at all - a mutally supportive unit they become. All the threats and difficulties are external, and mostly historical - childhood friends turned bad, troubled childhoods, generational problems (multi-generational in Jasper's case).
Often the solution is a Deus ex Machina, including the one which terminates the book (and also incidentally in the case of the 50 year later coda bringing the tapes to light), and none of the problems they individually suffer seem to have consequences further down the line.
The occassional introduction of characters from his earlier books smacks a bit of laziness and a bit of self-referential wanking. The attempt at realism in this book really doesn't cut the mustard - whilst reading I felt that the author hadn't been there in in 1968 (I wasn't there either, but I was alive and 16 and aware of what there seemed like). It has obviously been researched, although occasionally I felt he had got the timing a bit out of joint - but that may be my memory of the sequence of things being inaccurate - but the research didn't seem to have uncovered the soul of the times. It added nothing to the thousands of pages already written about the era and the millieu. On checking after eading I discover the author wasn't born until 1969 (maybe he thinks he has Dean Moss's soul!).
The fantasy elements like Jaspers back story and the whole Jasper character are much better written - perhaps Mitchell should stay with fantasy, Cloud Atas and Bone Clocks all got six stars from me and Ghostwritten and Thousand Autumns got five, Utopia Avenue is far from those levels.