Like many, I was pretty skeptical about dropping this much cash on a .pdf. Since I have seen various random tables for OSR use before, I was also skeptical about just how useful this would be. But I watched the Questing Beast video review, which praised this product very highly, and showed some intriguing glimpses into the book; and I noted the nearly-universal high praise across the internet for this product. Finally, I took the plunge.
The book is great; although I game on a fairly tight budget, and could have purchased multiple old-school adventures for the price of this guide, I DON'T REGRET MY PURCHASE AT ALL.
This 'tome' is really awesome. Within one hour of downloading my file, I was already nursing brand-new dungeon design ideas that I had generated via the Tome's random procedures, but which also very clearly felt like they flowed from my own personal creative input. This book's tables are surprisingly rich and detailed, but they work together in such a way that they preserve plenty of room for your own personal touches. As others have noted, you can probably use this book to generate content on the fly during play, but its intended and probably best use is for advance prep, when you can combine rolling on tables with time spent deciding just what those cool/crazy results will mean at your table.
Here, I'll give you an example. The book offers tables for not one but multiple ways to inspire an adventure design. Suppose we go with a location-based design. It takes about ten seconds on the appropriate tables to roll up an adventure location called "Sub-Pits of the Hollow Tribe." Wow, that sounds weird. Ok, but what is it? The book offers additional resources if I want to further define the villain, her/his nefarious plans, what the dungeon contains, etc. But it also encourages me to take some time to marinate on this idea...and before long I decide that the "hollow tribe" involves, quite literally, humans who have been hollowed out, to eggshell thickness, as their life-force has gone to support some Big Bad who lives below their settlement. And now I'm mentally writing up a new monster: hordes of "hollow" 1-hp minions who shatter into pieces at the slightest hit...but don't let them touch you...or they will start draining away YOUR life-force, too. Again, all this stems from ten seconds of dice rolling followed by time letting the weird image it provided stew in my head. Over and over and over again, this book provides incredible depth with probably hundreds of thousands or even millions of possible combinations of adventure ideas.
Let's try something else: designing a dungeon. The book offers very useful, very detailed guidance for selecting interesting features in a dungeon. What I'm about to show in no way exhausts the book's dungeon-building procedure; this is just the tip of the iceberg. With a few rolls, I come up with a dungeon split into multiple levels/areas, with (for example), the following names: The Burial Wells. The Sinking Catacombs. The Fragmented Memory-Chambers. Then I start rolling for interesting features. The Sinking Catacombs contains a "throne of the frog." Hmmmmmm. Now I'm mulling over ideas...and before long in my mind I have a dungeon adventure: people for ages have come to bury corpses by dropping them down into the ancient wells in the cave on the edge of town. There's a sleepy guard there but when he's not looking you could climb down into the wells. The level below is flooded (and probably ghoul-haunted, right?) and one end of the tunnels below is COMPLETELY flooded but leads to the next level. But there's a room down there with 3 thrones with frog-heads...sitting in the throne for a minute makes you grow gills, which will remain for 1 hour...allowing you to swim down the tunnel to the 'memory-chambers" beyond...which I decide were an archive for an ancient order of sages, who inscribed messages to each other on the walls as a kind of ancient bulletin-board service. And if you copy any of these ancient texts, they immediately vanish from the walls, but you can probably sell that ancient lore for a LOT of money. And...
And off I go. Again, just a few dice rolls plus time to marinate, and I end up with a passable framework for a weird dungeon that fits in with the campaign I'm just starting up. Of course, the same dice-rolls at YOUR table would lead to a completely different dungeon, despite the common point of inspiration. This, in general, is how the book works; it sounds simple, but it's flexible and very inspirational, offering freshness without replacing your own creative input. I highly recommend this book.