I can really see why folks are in a snit about this book. She's playing both ends against the middle and folks are missing the main point of the novel.
Firstly, she starts with the age-old trope of killing Hitler before he gets a chance to begin the Holocaust. I'm giving nothing away, it happens within 5 pages. That's a dangerous place to begin. Secondly, she's playing on the time-worn subject of the Buddhist "bardo" state aka purgatory aka reincarnation (not all the same things, really). I'll certainly admit to being distressed after the first lengthy loop in time - wait, all of that is now going to be rewritten? For reals?
She seems to be working these both into her novel to make it seem that it's about these two things - and it's these that readers are likely irritated by. But the book really isn't about those, per se. It's about her life as an Englishwoman, someone who clearly loves her country, trying to understand what it was like to live through the days before, during and after WWII. Cases in point - when she describes what she loves about the English countryside, or what London is now missing because of the bombings, or the unenviable task of picking up the pieces after a particularly bad air raid. She also does a commendable job recounting what was to be loved about Germany before the war, as well as the abject misery of its citizenry during the war.
In the end, it moves very quickly for a 500+ page novel that keeps repeating itself, and it's engaging in its description of England and Germany in the 30s and 40s. Reason enough to read?