The trick to the "Groundhog Day"-type story is to set down the groundwork and then, in future iterations, to hit just the highlights so that the reader doesn't get as exhausted as the protagonist typically does. I think Atkinson did a wonderful job of this, balancing just enough "old" information with the changes that mark each new version.
I have to admit, there were a few places in the book where I was teetering on the edge of abandoning it because it hit a few triggers for me. I'm so glad I stayed with it. For example, in the beginning, I was dreading how Ursula was going to die each time (again). But about a third of the way in, it became more about how events were going to unfold, as the timelines got longer and longer and differences started to pile up. Again, the beauty of the iterative structure is that the protagonist can "get it right" in the end. And get it right she does!
The book hit several "good" buttons for me, too: English setting, lots of World War I and II action, female protagonist and supporting characters, and the "what if" setup. But the best thing about the book is the writing--evocative, funny, poignant, British.
I took off one star because for most of the book, Ursula herself doesn't appear to have much motivation--things just kind of happen to her, almost like she's a cipher. I was bothered by the fact that it takes her brother being shot down and presumed dead
for her to become proactive and start "driving the bus." You'd think that the events she goes through before that would be enough of a call to action.