This is an amazing add-on for an already amazing game. The philosophy on display is just as good as the original base game, though instead of carrying primary themes discussing machine intelligence and what it means to be alive, this DLC draws on Plato's Allegory of the Cave and translates it into a modern retelling with a twist. I won't spoil anything, but if you're thinking of buying this for the philosophy, it's well worth it.

If you're thinking of buying it for the gameplay, it's deserving in that respect too. They introduce no new puzzle mechanics, but rather take the puzzle mechanics of the original game to their logical extremes, making for mind numbingly clever and difficult gameplay that does a lot of interesting things with mechanics you're already familiar with.

It's short, but has a lot of extra content aside from what's required to beat it. You should also not buy this unless you've beaten the main game and want more. There is no tutorial here, the story is based on the ending of the original game which will spoil it if you didn't experience it already, and the difficulty is high. If this sounds good to you, give this a try. You won't regret it.

This Addon is not just "more of the same". For the puzzles they use the same puzzle elements, but every puzzle in this game is harder than every puzzle from the original (and those were already really hard in the end), BUT they are never unfair or too crazy and in this game they have this open design which allows you to freely choose in what order you want to solve the puzzles, in never got frustrating for me at least.
It's also not just more of the same in terms of story. In this Addon we see a completely different part of the simluation with new characters and mysteries, and so far it is really interesting and compelling.

Considering this Addon only cost 15€ it is DEFINITLY worth buying if you like the main game.

Great expansion to an already great game. New puzzles are quite difficult - very nice, Croteam doesn't mess around. This will blow your mind numerous times and give you many "a-ha!" moments. Script on terminals is as always excellent (thanks I assume to Jonas Kyratzes). You don't have to read it all, but if you enjoy good writing you'll be rewarded for doing so. This is quality, well worth the $15.

A truly magnificent expansion, very challenging and great atmosphere.

It's a quality DLC with great story and really well designed puzzles. Must have if you like the base game.