I have to be honest in that I know very little about World War I, probably because I'm American. In comparison to WWII, "The Great War" doesn't play on the American psyche as much because the United States entered the war very late nor was it a major factor in the Allied victory. Though To End All Wars doesn't come right out and say it, WWI was won by the British and more specifically their naval blockade which led to the collapse of Germany from within.
Now if you are looking for a book about military tactics or how the battles played out or even how the overall strategy of the major powers worked out, this is not the book for you. Instead, Horchschild track's many individuals, almost all British, through the war and shows the connections they have with other. The connections are interesting in that some are ardent socialists or suffragists related directly to the imperial war establishment figures, brothers and sisters and husbands and wives on opposite sides of the political fight.
And while there is some writing about the general misery and hardship of the soldiers on the Western front, a good bit more of the book is spent on the hardship of the conscientious objectors imprisoned in Britain. Of the specific plights of soldiers, perhaps the books most tragic story is that of the British Bantam soldiers Joseph Stones, John McDonald, and Peter Coggins. Each was a victim of shell shock convicted by the British army for dereliction of duty and sentenced to death by army generals, most of whom had never witnessed shelling or even been to the front. But the story of these soldiers only enters into the web of connected individuals through the conscientious objector Albert Rochester, made to dig the holes for the execution posts.
The importance of World War I is probably understated in the narrative of modern world politics. Not only did it setup the circumstances for World War II, but of more importance it directly led to the introduction of communism on the world stage (by the time WWII broke out, Stalin had already killed more of his countrymen than all the victims of the Nazis). If you are interested in "The War to End All Wars", then this book is a great companion to other material. But as a single read on the subject, I do not find it compelling.