Look closely. It's not trying to be Onimusha.
If I was a developer, my main concern in making an action-adventure game like Genji would be to make sure the combat had simple enough, tight controls that assured you that you just killed 20 guys single-handedly because you rule, not because the enemy stinks. And I don't mean making the foes "Ninja Gaiden"-level difficult...I just mean making the player learn a few well-timed awesome-looking moves that never get old and pitting you against bigger and scarier looking enemies as the game moves on. Seriously, that was all Genji did about the combat, and it can be just as satisfying as grinding through hordes of ridiculous baddies, sweaty palms and all, from a much more intimidating game like God of War or Devil May Cry.
You can choose between two very different but equally functional characters: **** who rolls, dodges and delivers quick slices of dual katanas, and Benkei, a slow, walking tank carrying a gigantic club. You can switch characters during the game, but you're almost never forced to. They have independent character levels, equipment and statistics that you boost separately, making for lots of leveling up you can do once you've beaten the campaign. And this is combat that makes leveling fun.
Most notable about the fighting is the Kamui ability, which, with the ever-filling special gauge and the press of the L1 button, lets you enter a time-slowing, zen-like focus mode. Once you've activated this, all the enemies around you will back up, then one by one, as if in an old Hong Kong movie, charge at you, ready to strike. All you have to do here is wait for a Square icon to appear below your character, press the button, and you'll dodge the attack and deal a killing blow. You'll keep this rhythm going until either you miss the window and get hit, or you kill everyone in sight. This is almost as easy as it sounds after a while, but taking out 7 or 8 guys in quick succession like this makes you feel very, very awesome. Obviously, with tougher enemies later on and shorter counter windows, it gets harder, but with more Kamui gauges gained later on, and as fast as it charges up, Kamui becomes less a special and more the actual combat system in later areas. Believe me, it just doesn't get old. You can even use Kamui on bosses if you're quick enough, though it just deals heavier damage instead of instantly killing them.
Throughout the game, there are weapons, accessories and armor for both characters to find and use, areas only one of them can reach, and well-items to find and give to the local blacksmith for access to even cooler weapons. Once you've done it all once, it'll be a while before you come back to the game, but it will still be as fun as ever. Throw in some impressive visuals and authentic far-eastern music, and you have a game of higher quality than most give it credit for.