I'm sure a thread was made about this a long time ago, but it seems to have been lost to the void of time and I can no longer find it.
Anyway, It seems relevant to post this now, with the game coming in like 2 weeks.
There is currently a demo available for download on the Xbox live network and PSN. If you sync the demo to your social media accounts (i'm guessing Facebook/Twitter) and defeat the boss at the end, you will get a Siegfried Systema Garb, based on Siegfried from Final Fantasy 6.
About the game itself, it is the final entry in the XIII series, and revolves around Lightning who awakens from crystal sleep after 500 years to find a world which has been changed since the events of XIII-2. She discovers that the world is ending in 13 days and is given a task from "God" to save the souls of the worlds population, becoming "The Saviour". Nobody ages any more since the events of XIII-2, and therefore returning characters still show up.
Gameplay wise, the game uses a mash up of the combat systems from XIII and XIII-2, mixed with the Garment Grid/Dresssphere system from Final Fantasy X-2, and you have complete control over Lightning, like running around the battlefield and performing actions like guarding with her shield, though you still have an ATB gauge.
Paradigms of the previous games are replaced by Systema, essentially a costume change. Lightning can change costumes or "Garbs" in the middle of combat, and each Garb she uses has a different look (which is colour customisable) and different abilities which can be customised as well. So it's like a super-advanced version of the Garment Grid system.
Here's the main battle theme, which I think is excellent and very similar to the battle theme from XIII, but made more dramatic:
And a look at the Opening Cinematic, which being from the opening, doesn't really spoil anything.
Oh, and a gameplay demo of course: