SpaceChem is one of those puzzle games people will be playing long after development has stopped, which is happening no time soon.
This is one of the hardest and most satisfying coding puzzle games out there.
Pros: -difficulty of puzzles leads to a huge sense of satisfaction when completed -lots of levels -you get items in TF2 -subtle chemistry education -engrossing, if arbitrary, story -helpful tutorials
Cons: -music is repetitive -graphics can be a bit of an eyesore -lots of levels -new players will never be able to figure out the last two Moustachium puzzles, as they're infinitely harder than the two previous -Fishcake is a bit dry -got Australium poisoning from handling so much raw Moustachium
But really, this game is fantastic overall. Get it on sale if you can, and make it a personal challenge to beat it. The rewards will speak for themselves.