One of the most challenging and interesting RPGs I've ever played.
The battle system is perfect. It gives you enough freedom to move, but limits you so that the battle is still a challenge. You must strategize in order to defeat the enemies and not simply spray and pray. However, I found myself rage quitting a few times because of how badly I was getting beaten. This is not a weakness, though. It means the game is challenging.
The visuals are another of the game's strengths. When I first watched the mitochondria transform a rat into a hideous beast, I was instantly in love with the game. The colors, textures, and lighting give the game the perfect atmosphere.
The characters are immensely interesting. There wasn't a single unnecessary character, and each person furthered the story in some way, whether they were on the screen for five hours or five seconds. The villain, however, is probably the most unexpected and interesting villain I have ever come across in any game. Eve is ruthless and cruel. She will stop at nothing to get what she desires, and she is pretty much unstoppable considering her weapon is inside every human except the main character.
The story, however, is the greatest part of the game. From the beginning of the game, you know something fishy is going on with the main character, Aya and the villain Eve, but the game sets up the story so perfectly that the player is unable to decipher what is going on until the game finally reaches its climax. That is true story telling skill.
Overall, the game is harshly challenging and wonderfully fun. I would recommend it to anyone.