The portrayal of wartime Britain is relentless and in the end wearyingly sentimentalised, the history students too busy being caught up in the moment to reflect on what they are doing there and what they might learn. There is an awful lot of running around and missed communication, and then the book ends in mid-story, without even the dignity of a decent cliffhanger, the publisher expecting you to buy the next volume to see how it ends. I will, but will wait until it is available as a second-hand paperback.
I was interested to note however that some of the errors of detail mercilessly catalogued by drplokta here and here seem to have been fixed - in particular, I was looking out for references to the Victoria and Jubilee lines and didn't spot either. The version I have is the free ebook supplied to Hugo voters, so perhaps it has been revised in the year or so since his notes.
Of the time travel stories, this doesn't annoy me as much as Fire Watch, whose errors of setting make it almost unreadable for me, but it is much less enjoyable than To Say Nothing of the Dog and a far less good novel (at least so far) than Doomsday Book. I shall do a post ranking the Hugo nominated novels tomorrow but you can safely assume that this will not be top of my list.