Brutally difficult, but definitely worth a play...
For everyone else, please check out this game if you can find it.
Ikaruga at it's heart is a simple 2D space shooter, akin to the ones that used to be all the rage at the arcades. However, Ikaruga adds a gameplay twist that makes it step out from the rest of the pack, along with upping the difficulty a few notches. There are light and dark colored enemies, and your ship itself can switch between these colors on the fly. If you are shot by an enemy ship of the opposite color, you die, but if you are shot by a ship of the same color, you gain power for your homing missiles. The catch is, however, that you do twice the damage to your enemies if you are switched to the opposite color. This sounds a little complicated, but you will quickly get the hang of it, and will soon be switching colors almost constantly. This color switching mechanic adds a lot of depth to what could have been just another side scrolling space shooter, and it makes the game actually have a very easy learning curve while maintaining an extremely high level of overall difficulty.
The graphics in this game are breathtaking by 2D standards. Quasi 3D backgrounds accentuate the fast, furious, and sometimes very pretty action going on onscreen.
Ikaruga's sound is also worth mentioning, as it is much more orchestral than many other 2D shooters. This really lends to the epic feel the game gives.
Ikaruga only has 5 levels, which may seem like not very much, but you will not be blowing through this game in one afternoon, even on Normal difficulty. The levels are hard enough that you are going to need to memorize them in order to complete them without using an insane amount of continues, but somehow, this is never tedious.
Add to all of these features the ability to customize your view of the game from horizontal to vertical and the ability to post your scores online to compare to others, and you have a pretty complete package that appeals any hardcore gamer looking for a real challenge, or just to gamers looking for some nostalgic 2D space shooter action with some new bits thrown in. Definitely recommended.