They're not kidding when they say "Extreme"
Story-What story is there to tell? You command your battle ship through the amazing designed stages fighting your way through Invaders from Space. How simplier can it get?
Gameplay-As said, you command your ship, collecting power-ups and fighting bosses at the end of each level, through space fighting off the space-infested Invaders. It's really quite satifying, actually.
Graphics-The graphics will REALLY sink in. Just think of it as a nice upgraded version of Space Invaders. The background will catch your eye and you'd pay more attention to it than the gameplay. Enemies and the ships are really old-school, just how we like our Space Invaders.
Music-They added a new feature to this game. While the nice robo/techno music plays, you also have control of the music. Everytime you shoot, you play a sound. Everytime you hit an enemy, you create music. Everytime you hit a shield, you play a drum. Its really cool, and its better you try it yourself.
Multiplayer-Not that you really go up against someone. In WiFi, You just submit your best scores up in the scoreboard.
Flaws-Well, when your in combat with the Invaders, the top screen is really quite useless, with gauges, meters, and whatnot. Multiplayer could've been better if you'd go up against someone in WiFi.
Conclusion-Space Invaders Extreme is really worth it, and worth the bargain. And when they say "extreme," yea there not kidding. So, you want a nice shooter, with a nice touch to the classic, then go pick it up.