The study of war literature does not end with Fussell. Things went on in the Anglospeaking world and elsewhere. Now there are many essays on war and literature, some with sounder judgements on single authors and books, though probably not as well written as The Great War and Modern Memory. However, no one can deny the simple fact that we wouldn't be discussing the issue of war and literature hadn't Fussell published this essay in the 1970s.
Having said that, I'd like to point out what aspects of the book are dated, since other readers have listed its merits.
First of all, the purely British canon Fussell analysed led him to some conclusions which are highly questionable when one takes into account the French, Italian, German, American, and Austrian classics of W.W.I literature. Let me say it clearly: an essay on the Great War and how it is remembered which does not take into account Remarque, Barbusse, Hemingway, and Lussu, is definitely too parochial.
Second: the idea that only "plain" narratives are faithful to the experience of fighters is definitely naive. Hence Fussell's bashing of David Jones, who wrote one of the most fascinating war novels (In Parenthesis), and possibly his decision to ignore the Americans (Hemingway, Dos Passos, and cummings being probably a tad too modernist to his taste).
Third: sometimes Fussell's use of Frye is persuasive, sometime it seems a bit stretched. To me Frye remains one of the great critical minds of the 20th century, whatever the bigots of po-mo in US campuses may preach; but the idea that irony explains everything written in this century, and that the main source of this ironic mood/mode is the Great War is a bit too
simplistic.
This doesn't mean I consider The Great War and Modern Memory unworthy of attention. It remains a must-read for all those who want to understand British W.W.I fiction and poetry. But it should not be read as an explanation of what W.W.I really was, and it should be read with other, more up-to-date books, like A.D. Harvey's excellent A Muse of Fire, who also works on non-British texts and offers a much wider and persuasive map of the relationship between war(s), literature, and the arts.