To put it simply, Ikaruga is the best over-the-top shooter I have ever played.
I love the black/white polarities and the constant switching to make this not only a shooter, but a puzzle game as well. The difficulty is insanely hard, but that just provokes you to play it more.
Graphics:
For a game like this, they are beautiful. Richly detailed levels and backgrounds. Even yout ship and the enemies have a great amount of detail for a genre of games that really "shouldn't."
Sound:
The music is really good, as well as the sound effects. Not the best I've heard in a game, but still amazing.
Value:
Like I said, the difficulty and sheer "you're gonna get your butt kicked" style of this game will keep you coming back for more. It's only 5 levels, but it's 5 levels of commitment. It will take you a while to beat, and a long time to master, if you ever can.
Overall:
I know lots of people wouldn't want this game, but I think everyone should check it out. Give it a try. Ikrauga is just one of the best games I have ever played, and definately the best when it comes to top-down shooters.