I remember having read the first volume some time ago and recently I decided to read through the manga in the span of some days. The first narrative arch is great, nice and distinctive narrative hook, really well drawn. But it almost looks like it was meant to be a short two-volumes narrative, not the six it turned out to be. I feel like the characters introduced in the first two volumes are the only ones to get a proper development, while the ones introduced after, are almost an afterthought; some secondary characters don't get developed at all. The story itself is quite enjoyable; personally, as stated before,I prefer the concept of the first narrative arc, but I can't complain too much as it never got bad per se.
However, it feels a bit far-fetched at times, and some of the revelations lack coherence. Especially, it becomes more generic, losing most of its initial charm.

The ending closes the main plot point, but I feel like it was a bit rushed. It was, all things considered, satisfying, but after reading again the first two volumes I had higher expectations: somewhat of a missed chance.
I would still suggest to read the first two volumes, though.