Few books about World War I have anything like adequate maps, making an atlas indispensible as a supplement. This it not the best overall atlas for most purposes -- for that I nominate The Viking atlas of World War I -- but it has its strengths. The maps are all stark black-on-white and can require a good deal of study to interpret. And its coverage can be quite hit-or-miss; I defy anyone to figure out what happened during the initial German advance against Belgium and France on the basis of this book, for example. But Gilbert's annotations are usually illuminating, and he does cover many aspects of the war well. Worth having, but not as one's sole or primary reference.
One regrettable lapse that it shares with virtually all such books is lack of any source references for its data. Since it differs with some other books on some important pieces of information, this is frustrating.