The two books are really one novel (thanks, publishers, for getting me to pay double!) so there's no reason to talk about them separately.
They're also part of Willis' time travel series, although they're not advertised as such. I really wouldn't recommend starting with these books; I feel that a lot of the questions and criticism of these books that I see in other reviews stems from the likelihood that readers haven't read the other books in the series: The Doomsday Book, To Say Nothing of the Dog, and Fire Watch. At the very least, you have GOT to read Fire Watch before reading these books.
That said, the books are excellent. Blackout starts slowly, but Willis does a great job of gradually but surely building the tension and intensity of the story, working from trivial humor up to tragedy... (and the tragedy that can spring from the trivial) although it never gets as intense as The Doomsday Book. The pacing is the main reason why I feel that the book should not have been split in two. The end revelations also came dangerously close to getting too sentimental/religious for me... but I think they fall on the OK side of that line...
I found the representation of London during the Blitz to be completely convincing and memorable - I found myself saying, "wow, I didn't know...." And I've also decided that it is virtually incomprehensible that I've been to London over a dozen times, and I don't think that I've ever been inside St. Paul's cathedral. I'm sure Willis would think that was sad and horrifically neglectful.
For another depiction of the Blitz, which also focuses on its effect on ordinary individuals, I'd highly recommend Sarah Waters' The Night Watch.