No matter how long you play
Robotron: 2084, you can always get a little bit better. There are no fill in the game - no cutscenes, no weapon upgrades - just pure action! The only thing you can 'collect' on the levels are the humans that you save, and this is part of the gameplay, since you must prioritize between saving humans and killing robots.
Robotron is fair - every time you dies it happens because you made a mistake. The game requires 100 % concentration. A few seconds distraction, and you're dead. And the game got more kills per second than any other game I know. It is addictive, but unlike many modern-day casuals, it doesn't feel like a waste of your life.
It has its weakneses, but I think it is one of the purest and deepest twitch games.