I have played pretty much EVERY RPG for PS2... this games trumps most of them. (I am partial to the Artificial Intelligence system in Final Fantasy 12) Nocturne is a very in depth game. You go through the game defeating demons after the destruction of Tokyo. However, you also get the demons to join your party, which is pretty simple to do, and then fuse them together to create more powerful demons to fight by your side. Most RPG's you get 5-8 characters who fight with you throughout the game. Nocturne gives you hundreds, literally. It's always new, always exciting because your new demons all have different abilities and levels of strength and magic. It NEVER gets old and dull. The story is OK, but this game is really not all about the story. The game is also HUGE. There are tons of levels and different areas to go through.
I think the main thing that makes this game harder is the fusions. Not having the right spell in this game can be your down fall, and if you fuse you only monster with a healing spell, you're screwed. I'm used to the fusions because of the Persona franchise. Granted, they aren't exactly the same when it comes to the monsters but the same principle is there.
Even for seasoned vets, I'd recommend you at least look at an online guide if you get stuck or frustrated. This game doesn't have many clues when it comes to what to do next at times. If you take a break from this game, prepare for a lot of wandering until you remember what you have to do next. The pros to this game is the challenge. You can depend on having to look for weakenesses when it comes to bosses. The story line is great. I'm about 20 hrs in. Basically you control the destiny of the world. Your actions will result in the various endings in this game. The graphics aren't anything special but they were good for a early ps2 game. The music is pretty good, all of it is fitting for the game.