Onimusha 2 is like a watered-down version of Ninja Gaiden. It has all the basic elements, it takes place in feudal Japan, and the Onimusha series even has a historical context (even though there are extreme anachronisms like rockets and robotic submersibles :). Both games have excellent production qualities overall (despite of the truly horrible jap-rock song in the main menu *shudder*), and have grand storylines and dramatic and beautiful FMV sequences involving ninjas and demons.
But when push comes to shove, both games are all about fighting. And that's where Onimusha falls short. Onimusha 2 has 5-6 different melee weapons, depending on your thoroughness, and each has surprisingly few moves. And although you can unlock more moves with secrets, you don't exactly trip over these during your first playthrough. Coupled with the fixed camera angles, that comes with the prerendered backgrounds (as in Resident Evil 1-3), the battle system may be annoying at times. Overall, it is very satisfying, albeit a bit shallow, but when you reach a boss fight, you will be frustrated.
And that's another problem; on the normal difficulty setting (which is the hardest one accessible during the first playthrough), the main part of the game, where you kill hundreds upon hundreds of demons, the fightinh is almost too easy. Unlike Ninja Gaiden, you are not forced to learn advanced techniques (like the counter-moves called Issen in Onimusha), and you'll just fall back on your basic hack 'n slash, which works just fine. But the game has 5-6 real boss fights, and here the difficulty ramps up to vertical. So if you're like me, you'll have to fight each boss a number of times while you actually learn the more advanced techniques of the fighting system, and the game doesn't have continue points like God of War, only save points, so you'll have to run the savepoint to the boss every time, which gets tedious. And a note of caution, if you are going to play this game: when you reach the big flying action sequence near the end, do try to conserve your health and magic items for the final boss battle with Nobunaga, which is insanely difficult, if you didn't.
The replay value of this games is tremendous, as you have multiple storylines with different characters, minigames with frog-stabbing, the lot. I, however, don't care, because I'm not going to play it again. I'll try Onimusha 3 instead :).
... I just looked through the concept art by Keita Amemiya, and it is very nice. He had a lot of great ideas that never made it to the game.