I just read Nightmare of Nunnally (in one night BTW) and have some thoughts...
Nightmare of Nunnally is an alternate storyline for Code Geass -most of you probably already know that. Overall, this is an excellent manga and I would recommend that every Code Geass fan get their hands on it or read it online.
I thought is was better than R2 by far even though it had some flaws.
Some awesome highlights:
* The irregulars are very cool - manufactured geass users but with a negative side-effect.
* I loved the reason why Nunnally suddenly couldn't walk or see (notice her eyes are always closed - blind people don't do this). She did it to herself because she couldn't deal with reality. (Although, I think the anime series was pretty much the same, right?)
* Loved the end where Nunnally has to grow up fast and make her decision at the doors.
* Knights of Round being undead - totally awesome!!!
* I definitely like Lelouch and Suzaku working together as friends better than as enemies.
* And finally, Euphie and Kallen are much less annoying than in the anime series. Euphie is way too soft and Kallen is way too loud and obnoxious in the anime.
Some minor criticisms:
* I thought Marianne and Charles were portrayed too kindly. You would have to be very EVIL (or very stupid) to want to kill God! But I guess since they were so arrogant, they thought they were doing the world a favor. But it seemed like Charles just wanted to BE God. (Note: The story proves that you can't kill God in Code Geass. Otherwise, why is it Nunnally's choice? Why do they need Nunnally? Obviously, God already knows she will do the right thing, that is why she is the key.)
* I thought the part in the last chapter about the hatred of the people toward Nunnally was kinda dumb. If all the areas were liberated, why would people still fight? Maybe because they still hold a grudge against royalty? I'm not sure why the author added that part. If the Britannian tyranny is ended, the conflicts would end (except for terrorists) because people only fight when they have no other choice.
* The story was a bit vague about the Lelouch/C2 merge and what exactly happens to him in the end.
* The manga doesn't really stand on its own. It is much better if you have seen the anime series. I can't imagine someone understanding the manga very well without first seeing the anime.
* And last, there is no romantic stuff in the story at all, which made it feel like I was reading a history text. As a girl, I like a little bit of relationship stuff or, at the very least, some sex for cryin' out loud! Even stories geared towards guys (James Bond!) have a little bit of something in them.
Anyhoo, sorry for rambling on...