I'm really liking this game, despite the slight repetitiveness of the arenas and enemies. Yes, this is a boss rush game, but the solo mode is quite expansive and with a lot of narrative to keep you intrigued. The reason they can do so much narrative is likely because it's not told through CG cutscenes, but with voice actors narrating the pages of the magic book in your cell, Librum, as you flip through it. Sometimes you'll have stylized art to go along with the narrative, and sometimes moving pictures, and all the while you'll have the beautiful orchestrated score by Yasunori Mitsuda and Wataru Hokoyama changing with the tone of the narrative.
Even though I don't understand the Japanese quite well enough yet, it's very engrossing.
The arena variety grows as you progress through the story mode, so you don't run the risk of getting bored of everything before you're half way through the game, and the enemy behaviors and combination is good enough not to be an issue. It's also a loot game in that you receive spells after every mission, especially if you get a good score. Then you can level them up and combine them, which is fun, and finding that perfect load-out for that one boss you've been struggling with is awesome.