Havent even heard of it but i checked it out, might even have to get the game.
It's considered one of the hardest modern games available and rewards smart thinking. If you play it like God of War then you will die, a lot, and fast. It requires slow, methodical movement and technique. You have to sort of re-write what you understand about RPG/action combat in games, re-program your brain into a new way of playing. It's hard to explain, but it's an amazing game.
Here's a few quotes on it from wiki that pretty much explain the reason why it's so good:
The game is known for its considerable difficulty, which has prompted much interest and discussion.
The challenging nature of the game provides a foundation of achievement and reward, reinforced by penalties upon player death
The combat is precise and crisp, it's pin-point. You swing only when you know you can hit and if you have enough time to shield up and dodge away. Normal enemies that walk around can kill you in 2-3 hits, and by normal enemies I'm talking fodder you would normally kill 50 of in a group in any other game going from room to room. Bosses normally 2 shot you.
The difficulty is skill based, not cheap. If you properly learn the attacks of enemies, learn your weight/roll speed/weapon type (very important things), and fight smart, you can normally have no problems with normal enemies and should only be fully challenged by bosses.
It's a system that rewards learning and playing intelligently, and penalizes any mistakes, I'm talking 1 mistake 2 minutes into a boss fight and you are dead, it's extremely rewarding and a great challenge.