Why haven't you played SpaceChem? Probably because you are intimidated by the trailer and think it's too complicated, or because you didn't know it exists. Well, this game does exist, and now you don't have an excuse anymore to not at least try the free demo.
SpaceChem is one of the best puzzle games I've ever played. It uses fake chemistry only as a backdrop for its puzzle mechanics. So don't worry, you don't need to know anything about chemistry in order to play this game. Your job is to design machines that transform a given input into a required output. For example, the game gives you single hydrogen atoms and demands hydrogen molecules (two hydrogen atoms bonded to each other). You can then use functions like grab, drop, input, output, bond, etc. to create a machine that automatically turns hydrogen atoms into hydrogen molecues. How you do this exactly is up to you. It can be very satisfying to watch your solution run, because it is YOUR OWN creation. You are not simply following a strict path the puzzle designer wanted you to take. And after each completed puzzle, a graph will show you how efficient your solution is compared to others, giving you extra incentive to make your solution work just a little bit faster or use less functions.
As you progress, more functions will be available like fusion or flip-flop switches, allowing you to create even more complex machines. Some levels let you link several machines together, so the output of one machine can be used as an input for another machine. The last couple of worlds are very challenging, but even if you give up there, you'll get your money's worth. And if the campaign isn't enough for you, there are still tons of user created puzzles to solve and challenges to master.
If you like puzzle games, or if you like programming, buy this game now! If you are uncertain, play the demo. It's pretty long and gives you a good look into the game. For some people the demo might even be enough.
I have finished the GOG version, and that's the one I'd recommend, because it includes the DLC.