Much like many other manga based on visual novels, they become mysterious beasts that become hard to define as to whether they can be considered 'good' or not.
I am writing this review as someone who has seen the anime adaptation and read the manga but not played the original source material, 11 Eyes the visual novel.

The manga follows a specific route of the visual novel, the Kusakabe Misuzu route.
As such certain plot points and characters are completely glossed over in the manga, as to understand the entire story one must play through all the routes of the visual novel.
The first chapter or two can be a real chore to get through, as it seems to throw concepts at you from the get go and you have to accept that things are happening blindly in order to enjoy it. However after struggling through them, you'll probably start to enjoy the weirdness of the situation and the characters involved with it.
Aside from the hurried beginnings, the story is more of a puzzle trying to work out why these characters have been thrust together into this situation, and what can they do to survive it. Almost nothing is explained until the last 2 chapters, where expositional dialogue will try and help you muddle through what exactly the hell is going on.

The artwork is above average in that for the most part in manga today i see little to no shading, vast amounts of white space and little to no detail. 11 Eyes will not disappoint with it's array of distinguishable characters, detailed fight scenes and a surprisingly heavy use of shading.

The characters are the real meat of the manga here, and in some parts this could be considered the best or possibly the worst part for other readers.
As previously mentioned, this follows the route for one character.
As such the character that you follow is well fleshed out, revealing backstory and seeing the emotions that she displays at any given time and her plights, hopes and dreams.
However the rest of the cast is left pretty much in the dust.
Aside from knowing who they are, the rest is left to either watching the anime or playing the game if you want to fill in the gaps.

Overall i'd say i enjoyed reading the manga, but mainly to get a different perspective than what the anime provided.
If you are already a fan of Kusakabe Misuzu's character, i'd definitely recommend reading it!
However for the rest of us i'd recommend watching the anime at least, to get a better understanding on what's going on with the rest of the characters and story. And for everyone else,including myself, 'id say play the game if you can and if you enjoyed that you'll probably enjoy this too.