As bullet hell shooters go, there isn't one as excellently realized as Ikaruga is. In this one, you are able to freely change the polarity of your ship between light and dark. Doing so allows you to absorb enemy attacks of the same color as you, and do additional damage to enemies of the opposite color. Destroying an enemy of the same color as you will produce suicide bullets (which you will either need to avoid or absorb). Absorbing shots charges a special attack, which once activated, sends a volley of polarized missiles across the screen.
This polarity mechanic takes a shooter that would have been excellent on its own, and makes it so much more.
If you're at all a fan of this particular genre of shooter, and haven't played Ikaruga, this is an excellent version of the game to play it for the first time. It lacks the online multiplayer which was found in the Xbox Live Arcade version, but is 5 bucks cheaper (and seeing how the XBL coop was a laggy mess, it isn't a huge deal for it to be missing here.)
What's added in this version is the ability to solo man two ships (if you think the game is a cakewalk as it is or something), local multiplayer support, as well as numerous screen rotation options.
It's a great version of what is quite possibly one of the best video games ever made! What're you waiting for?!