Spent the weekend playing Spacechem while watching the NCAA basketball tournament.  Though nominally about tearing apart and building molecules, its really a simulation of assembly line design, since you molecular engineering happens mechanically (ie carry atom over here, bond it in reactor, move it over there, etc).  There is a kind of built in re-playability, as most of the puzzles are not that hard to solve in some fashion, but can be very hard to solve efficiently.  For example, the level "No Ordinary Headache" will allow the player up to three reactors, but a one reactor solution is possible.  Took me forever to finally get it. This one is not mine but is not too different from my solution.

To that end, the game provides a distribution curve of other player's solutions based on three stats (number of process cylces required, number of reactors required, number of components required).  Even if you get the puzzle right, you may see you solution was way less efficient than other folks, driving one to try again.  I like this dynamic - it is sort of like duplicate bridge, where one is not judged by just winning the hand, but by how well one scored with the hand vs. other players playing the same hand.

Here is another positive review at South Bend Seven.  And just search "spacechem" in youtube to find zillions of videos of various game solutions, it will give you a feel for the game.