TL:DR = play with headphones, game lasts about 5 hours without rushing. didn't regret the 5~ euros I spent.
The game has an extraordinary level design with a really thick atmosphere. The beginning may be a bit slow, but the scenery will become wonderful once you progress.
There is not much to do. The core of the game is exploration and run / walk. There are very few puzzles (challenging enough) and some fast paced running section. In this sense, this game maybe would have had a benefit by implementing some kind of obstacle course runing style. Not really parkour, but at least a vault over object kind of jump and a slide (more or less like mirror's edge). Nevertheless, the fast paced sections are fun enough but sometimes the movements feel kind of funny and you will have to repeat the section.
It is not an action game, it is much more towards an inner thinking / interpretation kind of gameplay. There isn't even any lore, no text to read, no voiceover, you only get the name of the current level when you go on to the next level.
The only real problem I can think of and that was kind of gamebreaking for me, was that somehow the level would not load completely so you would go through a dark door and find yourself out of the map. I doubt that was a wanted thing, because if I reloaded the last checkpoint and played the same section, the same door would lead to a new room. Same thing with some 'interactable' (can't say more for spoiler reasons) objects: I would arrive at point X, and see that those objects were useless. I would be then stuck. By reloading the checkpoint (sometimes much much earlier....) I would notice how the same objects were actually supposed to be moving.....so well, yes, instead of having the surprise effect of seeing weird things moving in front of you, you have the hassle of repeating a whole section again to make the 'moving thing' script trigger properly.