I finished playing the demo for Ninja Gaiden Sigma 2 last night (available on the Japanese PSN with Japanese and English support), and it made me feel both better and worse about the upcoming release. 
 
The demo starts you out as Ryu in the first stage with a few level 3 weapons.  It plays just like the 360 version, until you get to about half-way through the first stage, which is where you fight a mid-boss new to Ninja Gaiden Sigma 2.  One you kill the mid-boss, the level ends and you unlock the ability to play as Ayane.  She has her playing style and her own level (which is based off of the Venice level).  It's a short little romp that ends after you clear a marketplace full of enemies.  Then you unlock team battle mode.  You and an AI teammate (one is Ryu, one is Ayane, you choose) fight in two different small arenas taken from different levels in the game.  Fight some enemies, have you partner heal you if you get defeated, do a combo Ninpo with a cool ending, and that's pretty much it.  You can unlock Way of the Master difficulty for the demo, and it replaces the regular enemies with harder ones and makes the boss harder to defeat. All in all, a good amount of content for a demo.  Now the good and the bad about it.

The good news is that it still plays great, the graphics haven't gotten any worse, Ayane is fun to play as, the team battle mode is fun, and the new level one mid-boss is a decent battle.
 
The bad news is that they have made the camera worse.  That was supposed to be one of the main selling points of this game was an improved camera, yet I would more often find myself getting hit by offscreen attacks.  Now, I personally had little to no problem with the camera in Ninja Gaiden 2.  The only problem with it was they took the near-perfect camera system from Ninja Gaiden and retained it without accounting for the faster gameplay of Ninja Gaiden 2.   However, in NGS2, I would have to struggle with the camera more, and it just felt closer to Ryu than the one from NG2.  Disappointing.
 
Another thing that was disappointing was the lack of gore, and I know that the fact that NGS2 would be less bloody has been common knowledge for awhile, but it's actually much more noticeable when playing the game.  The main reason behind this that they retained the ability to mutilate enemies.  I can slice off arms, destroy legs, and remove heads (all with appropriate sound effects)...and all I have to show for it is a bunch of purple smoke.  The game still has the quick kill attack when the enemy is missing a body part, but it just loses it's appeal when a bunch of purple smoke comes from a vaguely discolored hole.
 
Also, though I said that Ayane is fun to play with (and she is) I'd still choose to play as Ryu any day of the week.  Most of her moves feel like underwhelming copies of Ryu's moves and her projectile (at least in the demo) seems overpowered.  It's an exploding kunai and she has an infinite supply of them.  Also, the whole sixaxis boob-bouncing thing...overrated.  You have to shake the hell out of the controller for them to do anything, and then when they do, it's less like the boobs are bouncing, and more like they have become sentient and are trying to break free.  Looks unusual, and not in a DOAX2 kind of way.
 
I'm still going to get the game (in fact, I've had the special edition preordered for awhile now), and I am looking forward to it...but a little less now than I was before I played the demo.