Ikaruga is the spiritual successor to Radiant Silvergun and quite possibly the finest verticle shooter of all time.
The main concept of Ikaruga is that of polarity, you can switch shielding on the Ikaruga to either white or black. This enables you to absorb enemy fire if you are of the correct polarity, if you're hit by a bullet of opposite polarity however, your ship is destroyed. This simple mechanic enhances the game superbly, no more are challenges simply to avoid every bullet on screen but to time runs through a changing barrage of fire, gathering the excess energy and enabling the energy missiles to allow some breathing space. Having this mechanic in place demands quick reactions and precision from the player, giving the game a sharp difficulty for novices. The later levels turn into a subtle bullet hell (danmaku) type shooter, the only difference being that a button press will save you if timed to perfection. A repeated process of trial and error may be the only way some players can advance.
Graphically, the game is styled well. Polarity bullets aren't an eyesore, rather the opposite, creating an artsy feel. The levels all have nice backgrounds and the framerate, pleasingly and moreso vitally, is consistent. However, the aesthetics reach a high with the background music. The BGM for the first level (titled "Ideal") is so natural for a NAOMI (New Arcade Operation Machine Idea, Sega hardware) port and gives an incredibly heroic feel to the level. This wonderful track's legacy is present in the second stage and faintly in the third. However, for the final boss, it returns in a glorious fashion.
The major concern for me is that there is only 5 stages and the end of which, is a boss fight. Which is frankly very little by all standards. Don't be fooled however, for the difficulty curve is vicious (even if you tweak the settings) and the only way to pass the later stages might be persistence or sheer luck. A determined gamer should be able to get through it all but after that, there's not much replay value unless it's for admiration, nostalgia or high score attempts.
Ikaruga's genius and brilliance is no doubt in the simple inclusion of the polarity mechanic. Such a devilish trick that can make levels become so thrilling and cause fantastic strategies to be crafted by the player. This is no doubt the successor to Radiant Silvergun, and in many ways, trumps its legendary predecessor rather surprisingly. This is one game everyone should play, this game is a work of art, a masterclass that was so sadly overlooked and forgotten.