SpaceChem is one of my favourite games of all times, a 10/10, yet I am not sure I can recommend it. You need to be a particular kind of person to get into this game. And if you are not, you will definitely not enjoy this game.
This game is hard, very hard. I managed to finish it, but one of the last levels took me 2 weeks to solve, probably about 30 hours of trial and error, for just 1 level. It had me working with a pencil and a sketch pad as I sketched out little subsections of possible solutions. This is the only game ever that did that. The joy of finally solving it, was incredible, naturally, but you have to be able to put in the effort. Looking at a problem and going "ah, I'll just look up the solution on Youtube" is not the way to play this game.
At some point, every puzzles seems impossible, that is, until you solve it. And then the next seemingly impossible puzzle is presented to you. You can approach this in two ways, as the very best puzzle-challenge you will ever play, or as a game that makes you feel stupid. If you are the latter kind of person, do not get this game, obviously.
So, if you are looking for a genuine challenge that will push your creativity and out-of-the-box thinking to its limits, if you are tenacious and willing to be immersed so deeply that you'll start dreaming up solutions while you sleep, this is hands down the very best puzzle game you will ever play. If you are not, then this is definitely not the game for you.
Do I recommend this game? YES, just not to everyone.