Manga Intriguing
My curiousity was piqued by the idea of Nunnaly as a protagonist. She's just Lelouch's Morality Pet, right? So I picked it up and I'm glad I did.
Nightmare gives Nunnaly greater depth and she becomes more than just 'Lelouch's sister'. She takes part in events like the battle at Narita and we see what she would do if she weren't locked out of the loop like in the original. At the same time, Knightmare averts Adaptational Badass and keeps her the sweet girl she was originally, which creates internal conflict for her when she tries to forcibly stop the Black Knights and Britannia from fighting.
The story is different and the plot is different. You won't feel like you're reading the original from Nunnaly's viewpoint, but because it is Nunnaly's viewpoint some characters will be more involved in the story than others. Alice, for instance, is justifiably involved here because she is Nunnaly's best friend. Jermiah is not because he has no connection to her.
The world of Geass is fleshed out. There's barely any explanation in the original but here we learn about Eden Vital and the purpose of Geass. That Assimilation Plot Charles had in the original, the one that was introduced and resolved in about two episodes? Its the main plot here and its success or failure is the story's climax.
Its not Lelouch of the Rebellion; its Nightmare of Nunnaly. Its less Magnificent Bastard and more The Messiah. From the first nail to the final choice its a good story, if you can accept that. Its definitely worth checking out.
Manga Overrated
Nightmare Of Nunnally gets rid of many of the elements of canon Code Geass I like most, while retaining those I like least. In this AU, geass powers are physical rather than mental, and the Chessmaster characters receive less focus, so the primary focus of the story is therefore superhero/mecha action (although a Thirty Xanatos Pileup emerges in the last few chapters). Interesting characters like Jeremiah, Schneizel, Li Xingke, and Rakshada are Demoted To Extra, and most of the fights consist of Lelouch, Nunnally, Suzaku, and Alice curbstomping everybody else. R2's nonsensical metaphysics are more plot-central and less well-explained here. There's much less moral ambiguity here; Suzaku didn't kill his father and he deserved it anyway, and even Schneizel is clearly a good guy here. Oh yeah, and there's lots of awkward fanservice involving Nunnally.
The series has its good points, but most of them are found only the last few chapters. This isn't worth reading.