The concept of the story is quite good. Promising material for an exciting thriller. But all in all it's poorly written. So many dialogues feel unnatural and artificial. Just like the characters and their relationships. The puzzles are "okay" I guess? But for me they feel half-heartedly integrated into the story.
The "gameplay" itself is really terrible. I am talking about the "story"-sections that are just wall of texts and more wall of texts. The presentation is a bankruptcy declaration to the art of visual story-telling. And don't tell me 'this is a visual novel. that's how this genre is made'. This is not an excuse for such weak presentation or writing. If that's the standard for this 'genre" than the genre itself is a bad (and probably cheap way) of developing games.
In addition if you don't play the game with a guide, you will probably have to play the same passages three or more times. This is just bad game-design.
So all in all I would like to see a movie or series adaption of 999, because the story is pretty decent. With a polished writing this could be really good movie / series. Uchikoshi himself said that he would like to make a movie or TV-drama adaptation of 999 together with JJ Abrams (I'm not sure about JJ^^).

Gameplay / Gamedesign: 2/10
Presentation / Artistic Vision: 3/10
Story / Writing: 6/10

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All in all 4/10

Games don't get as fascinating as this one. An escape room puzzle game that is just FILLED with pseudo science, conspiracy theories and wikidiving that Uchikoshi went on at 3am on a tuesday night. 999 is a world that brings up outrageous theories and plays with the idea of them all being real, creating this seemlingly fascinating and fun adventure that throws curveballs on you at every turn. The greatest strength of this game though, and Uchikoshi's writing, is writing the absurd whilst also anchoring it down in humanity and this is the reason why 999 peaks above all else in its genre. It's a perfect blend of the out of ordinary with just the ordinary, a fantastic mystery that ends on a high note. Love it.

The extremely low level of difficulty prevents it from being a good puzzle game, but perhaps more importantly it contributes to the strange lack of tension and intensity that should be very present within the game's scenario, best illustrated by the final "challenge." Although in my opinion the story goes one plot twist too deep, it's still entertaining to watch it unfold, especially thanks to the game's clever subversion of Japanese character tropes. But the non-notable gameplay causes it to leave your brain soon after it's over, and I doubt I'll ever play through it again. There's also the massive missed opportunity of the game not keeping track of how long it took you to complete each puzzle- trying to beat your best time would probably upgrade redoing escapes from just tolerable to actually fun.