Well, it's been long enough for me to pass judgment on Ninja Gaiden II, and here is my review in a nutshell: I wish I had bought Lego Indiana Jones instead, and I am going back to playing Devil May Cry 4 on Son of Sparda difficulty instead.
The easiest comparison to make is between NG2 and DMC4, so I'll mostly use DMC4, an enjoyable game for the most part, as my reference. The key to my replayability of DMC4 is that DMC4 loves me and wants me to be more awesome. As such, it has a lot of tools that are just plain fun, a concrete way to measure how badass you look, and every upgrade you earn feels concrete, enjoyable, and above all, useful. NG2, a largely unenjoyable game for me, wants to restrict me and constrain me, since most of the upgrades are hard to see and mostly useless thanks to the Move Funnel, where you essentially only use about 4 or 5 moves per weapon (this comes up often in games like Soul Calibur, where your character has about 20 useless moves and 5 good ones, while games where you have only 12 or so moves like Street Fighter usually have less filler).
Let's start with my first gripe, which is the camera. NG2 has a player-controllable camera, which would be great if less moves made it shudder and shake. As pointed out by many other reviewers, the most powerful move from the previous game, Flying Swallow, is strong but punishes you heavily by turning your carefully crafted camera angle into a steaming pile of crap. I can't count the number of times I've beheaded a ninja by counter-attacking with my move of choice, then realized "oh shit, I can't see anyone" and been suicide bombed by some legless goon. I don't fault them for this overly much - the DMC4 fixed camera system is inherited from the Resident Evil series, and it took them until around Code Veronica to get it right, so maybe in a few iterations they'll get the formula working.
But before the camera ceases to be a problem, the level design needs to improve. If the game took place in courtyards and rooftops and city streets, there wouldn't be much of a problem. However, there are way too many small, tight corridors in the level design, which causes the camera to bump against walls spastically and essentially restricts you to two camera angles, Looking Forward and Looking Back. Since enemies never come from just one angle, you're left with the Dynasty Warriors problem of the camera leaving gigantic blind spots. Dynasty/Samurai Warriors games mostly take place in courtyards and fields and large, open corridors many times the size of your character that minimize the impact of this - if you can't see someone, you run around and turn around so you have everyone in front of you.
I'm not done harping on the level design, though - levels in NG2 are designed obtusely, and often I spend a minute trying to figure out where the game wants me to go. I spent a lot of time asking myself questions like "I'm supposed to go through the fire and over that wall? How am I supposed to know that?" or "Everything looks the same, where am I supposed to go?" DMC4 has its problems with guidance, but it also tends to send the camera on long, loving pans of your next objective, and it tries to avoid the backtracking problem by giving you a relatively useful mini-map with a blaring "HEY IDIOT YOU JUST CAME FROM HERE" icon for your reference.
One last jab at level design: water levels in 3D action games haven't yet been pulled off well (please provide me with counter-examples that don't involve vehicular action games). Devil May Cry learned its lesson in the first game. No one liked the water level there, and they never tried it again - a decision I wholeheartedly agree with. Plus, it just looks silly when you put a hand-cranked harpoon gatling gun in your ninja game. After spending hours crushing bones, slicing off limbs, and tearing off heads, it feels silly to spin around in the water cranking a harpoon gun as if it were a '20s-era movie camera.
Enemy spawn patterns need a lot of work too - there are strong visual cues in the DMC games when enemies spawn, while often enemies will just sneak up on you and stab you because the camera is pointing the wrong way in NG2. While I don't mind this TOO much when the enemies are ninjas, it's just odd that I have no indication that there's a giant wolfman behind me trying to chuck the torso of his fallen comrade. The bosses and newly introduced monsters are especially bad at this, where you have to come out of cut scenes or load screens blocking or you'll eat a third of your life bar via a throw or Screen-Eating Attack that happens within a split-second of you regaining control of Ryu. Ranged fights are also a party killer, where you have to take some time out trying to find where those damn archers are, then trying to figure out if you can get to them instead of trying to use ranged weapons against them and slowing down the game.
All this negative commentary said, I still really enjoy combat in NG2 - but a lot of other factors drag it down. In short: too much time spent trying to look cool, not enough time being fun.