The first volume is highly psychological, and mostly a slice of life. It depicts the usual life of the main character. He is an odd guy, with fears so strong that, when alone, he mixes his dream world with reality. He is the "hero" of his own half-dreamy life. So, when the first hints of horror appear, they look like the outcome of his fears: is he dreaming again?
After this excellent beginning, the action and horror take a larger place in the story, but the psychological part doesn't disappear. Up to volume 4 included, the behavior, feelings and attitude of the main characters are an essential component of the series.
The turning point is volume 5. There is a major twist there, a smart and disturbing idea from the author. Unfortunately, this volume is also the beginning of a new arc: 40 pages totally disconnected from everything else, with absolutely nothing interesting. Just scenes of zombie horrors interleaved with an online chat. But, as the last chapter of v5 went back to the main characters, I thought these 40 pages were just a one-shot filler.
The volumes 6 to 8 were less original. Indeed, I felt the story was very similar to several classical survival manga like Metro Survive. Yet the art was still brilliant, and the tension was still there. While slightly disappointed, I was eager to read forward.
Then this manga seriously began to go astray. Half of volumes 9 and 10 were about the new arc, and the whole v11 went on. Why did the author build such detailed "heroes" to ditch them suddenly and focus on a gang of stereotyped characters? This new arc is plain and boring. No one has any depth. No psychology, very little action. Even Illuminati-manga wrote that "not much is actually happening" in the v11 they scanlated. I feel sad, as if the author betrayed the manga and let it die. I'll probably have a look at the next volumes, but just to flip through the pages out of curiosity. I have no hope this manga can come back from the deads.
EDIT: I've just read vol.12. Brainless action, with nothing original. The last 2 pages go back to the characters of the first arc. If it was not for that, I would have dropped this series. Now I have some little hope v13 will revive my interest on IAAH.