It has been a little while since I have read this book so I will keep it short.
The author's remit here is the European war from D-Day as experienced by the US forces. As such there is little material on the other allies, which is not a flaw (as some reviewers see it) because the book does not set out to cover the entire period or theatre. If it had covered the entire allied efforts in as much detail, it would be a hugely long work and as mentioned, not in it's remit. Ambrose was after all an American and few would criticise a similar book from a British, Canadian or even German point of view on the same grounds. To those accusing the author of a singular American outlook, I suggest his excellent 'Pegasus Bridge' an account of the British Airborne's skillful and brave opening raid on 'Fortress Europe', which he handles fairly and with suitable respect.
Ambrose gives the reader a good grounding in the main events of the period, detailing chronologicaly the American drive toward Germany from June 1945 to the end of the war in Europe. This wouldn't be the first book I would recommend to someone who wants to find out about what happened between 1939 and 1945, as it's scope is somewhat narrow, there are other books which cover the whole war more generally but Citizen Soldiers does fit in when the reader is more aware of a wider picture.
On a negative note, Ambrose does levy in a hefty dollop of opinion, which does detract from it as being a work of detached, unbiased and objective history. Sometimes the opinions are those of the participants, which is fine and is after all a part of history. However sometimes the author introduces his own opinion, which is not. If you can get over Ambrose's less than objective style, you'll find a worthy history, just make sure you are not expecting a full account, from all sides, of the last great efforts of WWII.