There are a lot of things I really like about this novel. It is extremely well researched and detailed. The characters are fairly engaging and no one makes me want to throw my kindle across the room. And I can see why it won the Hugo. One of the cleverest things about the book is the way that Willis uses time travel as a device to put a second layer between the reader and the events of the novel. But by embedding that layer within in the novel, it forces the reader to examine the nature of fiction and reading. In other words, because the protagonists are observers, and we the readers are also observers, the faults of the protagonists, in regards to how we/they view the events of the bombing of London, become those of the reader. It's an interesting experiment. Related to that, I also enjoyed the way she uses the time travel to show how history isn't static. The characters go into the London Blackout assuming they know what will happen and how things will go, but life doesn't work with expectations and even though to our characters these events have already happened, it doesn't mean the events will happen exactly as the characters have planned. Essentially, it may be history to you and me (or fiction) but to the people living it, it's simply life and life is never predictable.
However, while I enjoyed many aspects of the novel I can't rate it higher because the constant POV switches just bugged me. Willis has the bad habit of stopping one a character's story abruptly with a cliff hanger and then moving to a new character's story. That would work once or twice, but over and over in every single chapter it became annoying and detracted from the flow of the story. There are many of other points in each character's narrative that would have made much better places to pause their story and move to the next. I'm not fond of cliff hangers in general as I think it's a cheap way to execute the 'leave them wanting more' admonition, and this novel just had too many of them.
That issue combined with the fact that I found myself skimming the later chapters in order to push through the intricate, well researched, details to get to the plot brought it down from a higher rating.
One more thing. There are a lot of, OMG character x just died. No wait, just kidding. There were two characters, TWO, in all of the many dangerous times of WWII England, who died. And those two are actually more missing and assumed dead. I would not be surprised if they show up in the second novel all, whoops just kidding not dead. It sort of detracts from the idea that life, especially during a war, is chaotic and messy.
I'm not sure I'll read All Clear. I'm still debating if I care enough about the characters to find out if they'll make it home or survive the London Bombing. Especially since death doesn't seem to stick very well in this novel and I can predict the outcome.