The thing about Ikaruga that's hard to get when you first see it is how deep it is.
On the surface, it's a relatively short, extremely challenging shoot-'em-up, and is one of the last of its kind to have anything resembling a high concept. Most of the schmup genre these days are content to be purely based around bullet hell, where you're constantly ducking and weaving to try and find that one decimillimeter of screen space that isn't currently occupied by an enemy bullet. They're reflex tests for the obsessed and the young.
Ikaruga, conversely, is a puzzle game, albeit one in a weird sort of way. Your ship is always surrounded by a shield that's either white or black, and with a touch of a button, you can change its color. All the bullets, beams, and lasers that are fired at you throughout the game are either white or black, and your shields can absorb any bullet that strikes it as long as it's of the same color the shield is. Absorbing bullets is how you charge the game's "bomb," a series of homing lasers that can sweep the screen clear.
This isn't a new game. It originally came out on the Japanese Dreamcast a few years ago, and eventually made its way to the GameCube courtesy of Atari. When it was released, it was a bit of a cause celebre among hardcore gamers, mostly due to its pedigree. Ikaruga was developed by the Japanese studio Treasure, which has a habit of making idiosyncratic but amazing games, so it was a must-play from the word go.
As such, you can probably find quite a few videos of the game without trying very hard, displaying what high-level Ikaruga play is like. The first challenge you face is simply beating the game at all, which isn't easy. It's not just a bullet hell shooter; it's a conditional bullet hell shooter, requiring you to dodge, weave, and occasionally shift shield polarity in order to survive.
Once you've mastered that, the game requires you to have split-second reflexes and timing in order to get anywhere near an S rank on a stage. To get a chain bonus, vastly multiplying your score, you must destroy incoming enemies in alternating groups of three. The challenge is now to stay alive, absorb the right bullets, and kill only the right enemies. It's a reflex and pattern recognition test like nothing else in gaming.
I'm surprised to see Ikaruga on Xbox Live Arcade, because it's really not what I would've thought as a casual game. It's a hardcore game by a developer that targets a hardcore audience. I recommend it to those who grew up in arcades, those who enjoy a challenge, and casual gamers looking for something they can pick up and play. Becoming actually good at it requires a substantial time investment, though, so for all the elegance and style of its design, it can be remarkably frustrating.