Naturally, one's mileage will vary depending on what one expects to get from a time-travel story (thrilling introductions of tech to ward off the dark ages, nifty timeline-jumping alternate universe crossovers, plots to foil hidden history point-of-departure and make the world as it should be, islands in the sea of time populated by men who fold themselves, etc.).
The appeal of
Doomsday Book (which, incidentally, is one of my favourites) is not that it's a time-travel story, but rather it's a story about human relationships and the caring and concern we take for each other and the stresses that forge these bonds even between people we don't like and who don't like us either, which just happens to use a time-travel framework to express all these things.
The novella
Fire Watch (available to
read free online at the now-defunct-but-still-available Infinity Plus magazine) encapsulates all this very neatly in fewer words.
For what it's worth, I felt
Black Out and
All Clear dragged and suffered greatly from being basically one book that was forcibly split into two because it was cheaper for the publisher to print two shorter books than one long one (an industry change which also affected Lois McMaster Bujold's
Sharing Knife series and Peter Watts' third Rifters book,
Behemoth [split into
B-Max and
Seppuku in print form], for the worse).
I still enjoyed most of it anyway despite that. But it's not nearly as good as
Doomsday Book or
To Say Nothing of the Dog.