I had no interest in the deck or the other accessories, so I just bought the digital book on D&D Beyond, and I'd say it's paid for itself since.
- I've used many of the included stat blocks, magic items, feats etc. all over the place. One of the characters in a long-running campaign of mine is a charlatan Arcane Trickster who has been flicking playing card themed Magic Missiles around for years already, so he got plenty of new toys to play with.
- In another campaign I lifted the Heralds of the Comet faction basically wholesale and made them part of an ongoing Far Realm incursion metaplot. Their base became the party's Bastion once they'd cleared them out and the blonde human Aspirant who appears in the art was an NPC they callously allowed to be eaten by a cosmic horror and who will hence be returning - horribly transformed - as the campaign's Big Bad.
- Riffing on that connection to the Deck of Many Things, the same party had an adventure into the Shadowfell version of their hometown where all the creepy encounters were based on the cards as well as their own past escapades, and I used a lot of material from the book for that.
- In another campaign, the party will need to visit Baba Yaga's hut in the near future and I've used the House of Cards dungeon in the Void chapter mostly unchanged as the basis for the hut's demiplanar interior (it's all just made of rotting planks instead of cards). It's a surprisingly neat fit, actually - the three Fate Hags will be images of Tasha at different ages, and the Talon Beasts kind of look like they have chicken legs.
- Likewise, the Seelie Market will be dropped into the Feywild somewhere when they get there, and I've already used bits and pieces of it for the other group.
- The Sky of Many Things sits alongside Nyx in one of my settings that uses the Theros pantheon, basically as the secular interpretation of the constellations.
- I used some bits and pieces of the Donjon Sphere for a gnomish clockwork toy factory that was going wrong (pesky ceremorphs got in and nearly ruined Christmas by putting illithid tadpoles in the packages!).
I'm a great one for cannibalising setting and splat books, as you can see, so the Book of Many Things - which is basically just a grab-bag of (loosely) themed content - is ideal for me. WotC probably should have sold it on that basis instead of just treating it like an add-on to a fancy box of accessories for completists.