After watching the anime, I was desperate to find more of Shion & Nezumi's story, and discovered the manga was being published in Japan. The anime was only a half season and truly lacks much of the depth and story that the manga and novel include.
note: I have not read the official English translation of the manga. I am reading the Japanese versions.
Deep and intriguing, as if the premise of No.6 wasn't interesting enough already, there are many underlying and interweaving layers of the story. Things are truly not as they seem, but go much further than many other "ideal but not so ideal city" concepts there are out there. The story also leaves many opportunities for fan creations to be made - something I feel of great importance within anime and manga. As a result there are lots of lovely fanworks and doujins of No.6 to read and enjoy. I believe there are many parallels you can draw between No.6 and the real world and its happenings.
Along with each special edition of the Japanese manga, there are booklets called No.6+ which are little extra side stories to go along with the manga. (I'm sure you can find these online somewhere!) They don't run in tangent with whatever's happening in the current volume, but are just extra happenings in the No.6 universe. I personally love them, as they are often exploring and adding extra depth to Nezumi and Shion's relationship, and the world they live in.
The art for this manga is possibly my favourite of all time. Sharp, crisp and flawless, emotional and just wonderful. Kino has a wonderful way with expressions and eyes. As an artist I can really appreciate so many elements of Kino's work. As a fairly new artist, it's incredible she can deal with many types of panels so well - panels with action and fighting never become confusing (something that fighting and shounen manga often falls victim to) and the lighter, more casual panels are equally pleasing. In the No.6+ booklets aforementioned, there are full colour illustrations included that are just beautiful.
The biggest drive of this series for me are the characters. I truly love the characters in No.6. Shion, Nezumi, Inukashi and others too are fabulously written with depth and meaning and are inseparable from this story. After reading this manga almost in its entirety (volume 9 was released yesterday, and has not shipped yet), I really feel as though I know the characters, like I met them. There are very few manga that have given me this feeling after reading! No character was wasted, none are flat or worthless. Every character serves an absolute purpose, and without which the manga would suffer greatly. I also think that the characters in No.6 represent many things in the real world too.
I could not give this a full 10, as like with many manga, there are moments where the plot switches to the side characters, and while reading I feel like saying "Come on! Hurry up! I don't care about you, I need to see if Nezumi and Shion are okay!" and I would often skim-read or read super fast to get further. Perhaps that's silly, and perhaps that wouldn't be the case if I liked the side characters more, but I feel it's something to note! Other than this, I endlessly relish in this manga! I'm always finding myself picking up volumes and randomly flicking through them and just poring over the pages.
This manga is an exemplar of its genre and is perfectly written and executed. Beautiful art and well paced, deep storyline and wonderful characters. I guarantee this manga is worth your time!