SpaceChem is a puzzle game that deals with atoms and molecules.
I don't really think the story makes that much sense, but where this game really excells is in the puzzles.
The game is made up of chapters and levels. Each chapter will introduce one or two new new gameplay features. The levels take you through an easy tutorial of how to use the new element, and gets progressively more challenging. This learning curve is very good.
As for how the levels themselves work, you are given various tools and a grid. The job is to move elements (like hydrogen/carbon etc.) around, and join them to form molecules which you then pipe out. It initially seems easy but once you add mulitple reactors piping out to each other it quicky adds more of a challenge.
My only issue is, if I find myself becoming fixated on ridiculously complicated solutions. You can't skip levels if you are finding them too complicted. This means if you are completely stuck and can't progress unless you can solve each level sequentially.
This game is controlled with a mouse, but has keyboard shortcust to make things easier to control. This works really well, in my opinion.
The graphics are very simple, which is good as it makes it easy to see what exactly you're doing.
The music and sounds in this game are very nice.
There are extra challenges included once you complete the game, and ones that can earn you extras in TF2, it seems. This coupled with the wide number of challenges means the game definitely has a very good play-through time.
Definitely buy this game if you like puzzles, but don't buy it for it's chemical/scientific accuracy.