Tuchman's book is still widely read, so this question crops up here quite frequently, and it has attracted a number of worthwhile responses over the years. While there is always more to say, you might like to review some of those earlier responses, agglomerated here a while ago by u/scarlet_sage:

This in which /u/sunagainstgold critiques Tuchman's general approach. I think the other replies in the discussion are interesting but off the topic of Tuchman; it's talking about in-depth professional investigation and changing foci.

In "The Guns of August and new WWI historiography" , /u/Gimpalong addresses "war by timetable".

I'll try to answer your question.

First off, I should state my biases up front. I really like Tuchman's work. I've enjoyed both the Guns of August and A Distant Mirror. I know a lot more about WW1 so I feel like I can speak about Guns of August. I'm sure /u/sunagainstgold raises valid points about A Distant Mirror.

So first off, it's not as outdated as Gibbons' work about Rome. As I'm sure you know, those books were published in the 1700s, while Guns is from the early 60s. The cultural milieu of the 1960s and the work of historiography is fairly similar to ours, and a fair few books written in the 1960s and before are still cited by historians (pretty foundational ones, to be fair). The work also isn't plainly falsified.

In short, the main criticism levied at Guns is that it's old, and therefore some of the historiography is out of date compared to more recent works. The other main criticism levied at it is that Tuchman was a "pop historian", and therefore her work isn't 'real history', so you're better off reading a proper history book written by a professor/historian.

My opinion on the book is this: Don't read Guns to understand why WW1 started - it's not about that. Less than 1/4 of the book occurs before August 4th, and most of that is dedicated on pre-war military planning. The book is really about a very narrow place and time - the early military campaigns in France in August 1914 (with two digressions to Tannenberg and about the Mediterranean). In my opinion, it explains that narrow place and time brilliantly. I've never read a book which does a better job summarizing those early military manoeuvres, and if that's specifically what you'd like to learn about, I personally say read it.

Some caveats: Guns of August was written less than 20 years after WW2 ended, so it is fairly negative about the Germans. The book paints the Germans as marauding Huns and in fairly unambiguously negative terms. I personally didn't have an issue with this - I think most readers are smart enough to not read this uncritically. Also, in fairness to Tuchman, the Germans were fairly brutal in the opening campaigns of WW1. Also, when Tuchman is discussing the immediate prelude to WW1, she paints the military plans as a bit too rigid, and makes the argument that the military leaders convinced their political leaders that military necessities overpowered political ones. This argument has fallen out of favour in recent years, and most people argue that the politicians consciously decided to start the war, fully aware of what their plans entailed.

Other things of note. I've personally never ran into anything which I thought was downright false in the book. Tuchman gives her opinions on the actions of the generals, but I never felt she was unfair. She lauds Joffre and other French generals for their successes, and criticizes them for their mistakes (of which there were many). She begins her discussion of Sir John French by saying, "no one who has not been in the same position can judge." She then criticizes him for his failure to support his French allies, and she's not the only one. Max Hastings is much more negative on French than Tuchman in his book Catastrophe 1914, which is a more recent book.

I feel some criticism of Tuchman's book is unfair. I looked through some of the links posted below, and one caught my eye. It was this one, by /u/DuxBelisarius. I was a little baffled by it, because he disputes Tuchman's account of Joffre and cites Robert Doughty as a more reliable historian, when Doughty has published an article which almost exactly mirrors Tuchman's conclusions about Joffre. This may be too much detail if you aren't too familiar with WW1, but in Doughty's article, "French Strategy in 1914: Joffre's Own" The Journal of Military History 67:2 (2003), Doughty says that:

"Joffre refused, however, either to reveal his concept of operations to his political superiors and his subordinate commanders or to commit himself to a course of action... Moreover, he did not inform his subordinates until he published General Instructions No. 1 on 8 August.

August 8th is 5 days after the war started.

Moreover,

"In the final analysis, Joffre’s strategy suffered from several flaws, the most serious of which stemmed from assumptions that the Germans would not drive across Belgium deep into the French rear, and that they would not integrate reserve units into their leading forces. Joffre and his staff expected the enemy to advance through Luxembourg and Belgium, but they expected the Germans to weaken their center as they pushed toward Liège, Namur, and Givet".

As you read Guns of August, you'll find that Tuchman makes the same arguments.

So, in short, I think her work holds up when she's recounting the military moves of August 1914 in France, but if you want to understand the political background to WW1, read a different book, such as The War That Ended Peace by Margaret MacMillan, or Sleepwalkers by Christopher Clark.

The book is contentious, so I'm sure others will pop in with contrary points, and there's always more to be said.

Many consider Tuchman’s The Guns of August to be a classic work of history. Her writing is evocative and clear, and she wrote with an authoritative voice. Yet, this is not the whole story of The Guns of August and First World War historiography.

dissent from the viewpoint of another commentator in this thread who claims that First World War historiography has not changed since the 1960s. This could not be farther from the truth. (EDIT Misunderstanding on this point) The historiography of the war has changed substantially since that period. In fact, historians today often refer to the 1960s as when the old orthodoxy took hold in the public consciousness (think of work along the lines of A.J.P. Taylor or Alan Clark). You won’t find many First World War books from that era being actively cited in the same way someone might reference debates around the “learning curve” today. By the time of the 1980s, historians started to reconsider some of the views that were espoused when TGOA was first released. By the 1990s and the 2000s, the debates had shifted substantially since the 1960s. TGOA is rooted in a very different historiography than you’d find today.

This isn’t to say that The Guns of August is bad per se, but in many ways it has been left in the dust by its age. Nor can it be considered some sort of foundational text about the First World War in terms of the historiography, while you’ll see historians occasionally engage with it, it’s not necessarily because the book is a foundational part of the literature. It didn’t advance new arguments for its era, it helped popularize them. Its best to think of historiography that reading newer stuff will generally be putting you on a better foot than older stuff. You can eventually explore older stuff, but you want to make sure you’re rooted in the current conversations and debates, not the older ones. Think of it as a “reverse 20 years rule”.

This brings me to my final point, that even during the 1960s Tuchman was out of step with the historiography and conversations of the era. She wasn’t even necessarily popularizing arguments from historians contemporary to her. Here I will quote from a modern scholarly review of the book (“Fifty Years On: The Guns of August, Always Popular, Always Flawed” by Samuel R. Williamson, Jr. in The Sewanee Review, Winter 2013, Vol. 121, No. 1 (Winter 2013), pp. 163-166)

the reviewer in the Journal of Modern History (March 1963), praised its prose style but found the account based "only partially" on the best available sources [...] Another reviewer, in Military Affairs (autumn 1962), pronounced that the "serious military scholar, however, will find nothing here that is new either in facts or interpretations and will be struck by the author's significant omissions." The writer of this review, who read The Guns of August when it first appeared and cited it three times in a major study, has always regarded the effort as seriously flawed.

The first reviewer from the Journal of Modern History was a historian named Ulrich Trumpener. He continued in his review that,

Numerous inaccuracies and over-simplifications, notably in the discussion of prewar developments and Mediterranean affairs, must be ascribed to insufficient familiarity with the relevant monograph literature. [...] neither the Russian and Italian document collections published since 1918 nor the captured German government viles, a valuable new source, seem to have been consulted.

Trumpener was hardly the only reviewer to note that Tuchman had a flimsy grasp of historiography, another reviewer wrote that, “Forty years of historical research are ignored as are the hundreds of thousands of documents that have been published by the governments of Europe.” Oron J. Hale wrote

But what disturbs a student of the history of World War I, even more, is the fragmented treatment of the outbreak of war and the events of the first thirty days. The war originated in the Balkans with the assassination of the Archduke Franz Ferdinand by fanatical Bosnian Serb nationalists, and from a local crisis grew into a general European war through the reckless diplomatic and military actions of Austrian and Russian authorities […] All this is excluded with consequent distortion.

These flaws in the book were noted when it was published, and as time has changed and the historiography has moved on, they become even more pronounced. One of the major points in the book is that of “butchers and bunglers”, with Hale actively using that wording to describe Tuchman’s views. “Butchers and Bunglers”, as a school of thought, has mostly been left in the dust. This is a major sticking point, and an example of the ways that First World War historiography has moved since the 1960s.

In short, it’s really not worth reading in 2021. There are newer and more up to date books on the war, and that period of the war. First World War historiography has changed so much, and even then, Tuchman wasn’t entirely couching her writing in the work being done when she was writing. These flaws are not just being pointed out today, they were seen even when the book was published.