Much faster than Space Invaders Extreme, but just as fun to play.
In a few words: pretty good, actually. The main gimmick of Infinity Gene is that it "evolves" as you play it, starting out simply as a sped-up version of the original, and eventually adding larger opponents and boss battles until it transforms into a frantic pseudo-3D bullet-fest. As you play through the game and score points, you unlock special features, such as bonus stages, new music and sound effects, and new weapons, which are displayed as "branches" on the stage map (as befitting of the game's "genetics" theme, which also uses scientific terms such as "mutation" and "apoptosis" for its stages).
While the initial concept remains as simple as it ever has been – move around, shoot down invaders, and attack the UFOs for big points and power-ups – with enemies swarming you from all directions (and sometimes spawning from all directions, as well), you'll need fast reflexes and an attentive trigger finger in order to stay alive. Once you reach the second stage, your ship will be able to move in all directions – perhaps a first for the series, as all of the others before it restricted you to left/right movement. The normal game is 30 stages long, with the stages getting progressively longer as the game "evolves". Finishing the game doesn't grant you a grandiose ending (simply a "congratulations" and a dare for you to play again on a harder difficulty level), but the ability to put your score on the online leaderboard for the world to see is rewarding enough. When you think you've had enough of the normal game, you can try fighting your way through Challenge Mode, which is 99 levels long and never uses the same flight pattern for enemies more than once. Or, you can also try Music Mode, which generates one-off levels based on music stored on your device (Audiosurf and Beat Hazard have used similar gimmicks).
True to the game's roots, Infinity Gene is displayed with a limited color palette, with various shades of black and white for the ship and bullet designs and subdued colors for the background. This works out just fine for the basic stages, but when elements such as walls and large 3D enemies are introduced, it can be hard to see where you're going and what to avoid, as touching anything that's the same shade of white as your ship means instant death (though curiously, you can pass harmlessly through bullets the moment after they've been fired, which in game terms is called a "Nagoya Attack", and is desirable if you want to score a lot of points). Music consists of your standard shoot-em-up arrangement of techno-inspired tunes, most of which work for the stages they're used on, but aren't quite on the level of the Space Invaders Extreme games, which undoubtedly had the best music in the entire series.
Most of the people who pick up and play Space Invaders: Infinity Gene probably won't be old enough to remember how simple the games used to be (outside of the Taito Legends compilations), but those who have played the old games will be pleased to see how far the series have come since then. No matter what platform you own, it's definitely worth a download.