I have had this game for months now, and even though I beat it I am replaying it again on chapter Challenge.
I have been a fan of this series when it was on the NES and it was known as Legend of the Dragon Sword. Yes it can be difficult but when you pull off everything right with the moveset the rewards are very entertaining. Originally both Ninja Gaiden 1 and 2 were exclusively for Microsoft, but thank god they brought them over to the Playstation 3 since everything is all touched up and refined. Even though NGS2 is towned down on the excessive violence, i can understand why they did it, and actually I like what they did. The Gameplay to it is great, the story to it is good as well, the replay value is amazing since they now offer Co Op missions and you can play with a friend as well. The problem with random people doing co op is that there is no communication between you and him if they dont have a headset or use another program like skype ( i am very against using in game chat unless its a good friend of mine, but please use a bloody good headset thats wired because bluetooth really sucks for hands free headsets).
Also there's plenty of unlockables in the game, the weapons system is amazing, plus they added one hit kills when you amputate enemies as well. I recommend this one.