Nightmare of Nunnally appears to be a fascinating re-imagining of
Code Geass in the form of a manga series, but ultimately falls short of its promise and the impact of its source, presenting familiar settings with a paper-thin plot revolving around Nunnally and an expanded cast of prosaic manga-original characters that feel out of place compared with the colorful,
CLAMP-designed original crew.
Nevertheless, hardcore fans of Code Geass shouldn't be too disappointed with this manga series. Being one myself, I initially felt satisfied with it - hence my generous fair rating despite my prior statements. In retrospect, however, and from a critical perspective, it could have been a lot better.
[NOTE: This will not only be my review for this volume but the entire manga series at-large. This will be spoiler-free with the exception of one spoiler-tag-hidden bit.]

This image features Nunnally, her Mark Nemo knightmare frame and some old and new characters. The new characters featured in the bottom right with codes (the Geass sigil) on their foreheads are members a new faction existing within Britannia known as the "Irregulars." They aren't very intriguing and are My-HiME like in development.
For those already familiar with the series trying to decide whether they want to pick this up or not, let me explain the two biggest spins:
Nunnally gets powers akin to foreseeing the future as well as having a special knightmare frame (the Mark Nemo) under her command and
Zero is literally unbreakable with enhanced abilities akin to that of superheroes such as Batman and Superman. 
Zero is mad as hell and he can't take it anymore!
Essentially, the exposition of this manga series follows how Lelouch never comes home from the Shinjuku Ghetto (unlike in the original anime). Nunnally, in sadness and frustration, happens upon a doll lifeform known as Nemo who grants her a contract that will allow her to release her "wrath" towards her situation and the loss of her brother. This creates an alternate Nunnally who is perfectly physically capable and can pilot a special knightmare frame known as the Mark Nemo. The plot escalates similar to the original anime, with the Shinjuku Ghetto Skirmish, except plus the involvement of Nunnally.
To be honest, I saw a lot of potential with this plot unfolding in the first volume. Unfortunately, they fail to deliver thenceforth. The plot progression that follows is predominantly contrived, especially with the introduction of the "Irregulars," manga-original Geass-using Britannian girls in skintight suits. It's as bad as it sounds.
The "Irregulars" are practically used as plot devices with little-to-no individual development and are put to waste a lot like how the girls of My-HiME ended up in their anime (before its actual ending, anyway), hence my earlier analogy. While there is one slight exception, I did not find her exceptional enough to cover her more elaborately and reveal any spoilers regarding her in this review. To make an even more demeaning analogy about the Irregulars, they're like putty, filing gaps of the plot in an ugly way.
Even more critically, there are very significant twists from the original anime that this manga diverges from poorly. Beyond simply turning those twists over, they virtually present the most ridiculous, illogical scenarios that were poor excuses for changing the original twists. Predominantly, they do this in a certain case suggesting
significant preference for a certain character from the original series. [I'll elaborate a bit more in the spoiler tag below, for those aware of the original anime who are interested.]
Euphemia. She essentially even gets rewarded more than what she was campaigning for before she was done away with in the anime series.
For these reasons, this manga really is more like a fan fiction than an true re-imagining of the original series. Interestingly enough, by the end of the 5th volume of this manga,
the artist even writes an apology - admitting how his attachment to the characters (I even addressed this in the paragraph previous) adversely affected his approach to the
Nightmare of Nunnally.
This review covered all the negatives behind
Nightmare of Nunnally because I can only recommend this series to those who are very familiar with and love the anime it is based on -
Code Geass. The positives I have for this manga virtually relate with what I enjoyed from the anime series itself. To elaborate, this series is basically more of what we loved from
Code Geass, except more campy and less impressive. Otherwise, to an unfamiliar reader picking this up for the first time, this may be more of a "nightmare" than intended by its title.
If you love the
Code Geass universe, you should be able to appreciate this manga series despite its shortcomings. Even through everything I said about it, I was satisfied enough immediately after finishing it because I loved the franchise to the extent of enjoying more of it even at the cost of what made the original anime series so captivating. Perhaps this was because I never truly expected it to emulate the amount of impact its predecessor accomplished. Either way, it was a fun, albeit silly, read for me, managing to somewhat satiate my desires for more
Code Geass content.