Those without time to spare can experience the entire history of space schmups, several times over, in between bites of your sandwich.
Genetos is an "evolving shoot 'em up." The first level presents you with a very Space Invaders inspired theme, and as you progress through the game, it will progress through the decades, right up until some nasty bullet hell. Your ship will also evolve, giving you bonuses that compliment your play style.
It's easy enough to experience the last level, and fun enough to have at least a few goes. But Mac/Linux users should know that it's PC only.