I just finished the book, and I'm ready to give my (final?!) thoughts:
I think the biggest issue with the novel is that it spends too much time on the set up. Anyone who's read the book by now has probably seen S6 enough times that they don't need to be re-introduced to the characters, and it feels like wasted space to have long scenes establishing character dynamics that we should already understand. Jack's backstory was also a bit long and drawn out. A lot of time is spent elaborating on how the fog is affecting Sunnydale, which could've worked great, but there's so much, "the fog this, the fog that," that it grows stale by the latter half of the book. The last three chapters were good, but I wasn't as convinced as I wanted to be that this Great Battle was so big and earth-shattering. If it had been given more time and thought, maybe? I don't mind the foreshadowing to Dark Willow, but it happens so fast and would have been better if it was more subtle. Overall, the novel's pacing was the biggest drawback to me. That, and the crazy amount of typos.
I liked the chapters that we got in London 1888, I feel like those were the most well-developed and gripping, especially from the perspective of the Fanged Four. I honestly wish we had gotten more of that, because the book teases Spike's teaming up with Elizabeth, but even that plot point doesn't last very long. I grew to like Elizabeth's character and I think more could have been done with her. I actually think the Scoobies stayed in-character for the majority of the novel. I loved that we got to see Buffy, Spike, and Tara fight alongside each other, and I could easily picture shirtless Spike with a flamethrower 😏
The book is good, not great. There's just too much holding it back for it to be a great read to me, and even after I've finished it I just feel...meh? I'm glad I read it, as I've really enjoyed discussing it with everyone here. But I don't see myself reading it again. I don't want to discourage anyone else from loving this book though; it's a good Buffyverse story, just not my personal favorite.