I'll try not repeating to much, what other people allready said. But it's unavoidable. Anyway, here goes:
Ikaruga is a vertical scrolling shooter (refered to VSS below). Similar in the vain of Xevious & 1942 (arcades), and later Xenon & Banshee (Amiga), and thousands of other games of the same ilk.
This game is indeed, very old skool. Not only because it is a representative of a once very popular, but now, all but dead game genre. But also because the game structure hasn't changed one bit.
It has:
_ Only 5 short levels, that need to be played in order from the first to the last, again and again. (There are no save points.)
_ 2 Dimentional gameplay.
_ It's really hard.
_ A very basic front-end.
_ It's hi-score based.
_ A simple control-scheme.
Rather that changing any of the VSS rules or make-up, the developers have tried to perfect on these.
Ikaruga has a good and challenging level design. Some sections seem impossible at first, but with practice, these can be conquered.
The big improvement in Ikaruga's gameplay however, is the way the polarity, return fire, and chain systems work (read the other poster reviews for a further explenation of these). You can (try) perfecting those skills, and raking up that hi-score. They take the gameplay beyond memorizing the enemies attack waves, and continue to add incentive and fun, to replaying those levels. (One of Contra: Shattered Soldiers's (PS2 old school horizontal shooter) main faults, lies in the fact that there is no challenge anymore, once attack patterns were mastered, making for a boring game, really quick.)
The graphic design is also very good in my opinion, and very pleasing to the eye.
Shame about the borders, though! But there is so much going on, that I doubt a screen size twice the width would enhance the action. (Sure, you have the option of turning the screen display 90° to the right, thus eliminating those big black borders. But this isn't all that helpfull, since the tekst changes with it too, and simply, because the game is designed as a VSS, not a horizontal scrolling one.)
Also, it's a real pity, that there are only 5 levels. Even the old C64 games of this genre had longer and more levels.
Ikaruga is a real trip down to memory lane, with all the good and bad things that come with it. But thankfully, there is more good than bad to be found in this game. Overall, the (very small) Treasure team have done a terrific job with Ikaruga.