In my opinion, there are two different ways one can view this anime.

1. A plain old fashioned SoL with a healthy dose of mystery.
2. A redemption story (
   Spoiler, click to view

   reki's as well as rekka's
   )

If the anime had focused more on either of these ways and handled the distribution of information accordingly it would have been a 10/10 for me. However, it tries to be a bit of both simultaneously while skimping on the world building as well as some of the character's backstories.
Who are the toga? how old is the renmei? why are the haibane treated like second class citizens as far as their material status is concerned while simultaneously (and to my astonishment) being given the same dignity that human beings are given. Why are they confined within those walls? what is the significance of the ruins? How did they get there?
A lot of these questions are left unanswered and that would have irked me a lot if it weren't for the following:

1) The opening credits are pretty nice, the song is great and upbeat with a hint of nostalgia. The visuals are ordinary throughout the show however that did not hamper the experience in any way, heck it might have enhanced the mystery and psychological element of it. The ed was simple but captured the eeriness that the show had in its 13 episode run.

2) I was on the edge of my fucking seat for most of the anime because of the atmosphere that the visuals and the OSTs produced together. I do not understand myself how I managed to actually expect something thrilling or horrifying from a fucking SoL, and this is not me complaining, I am actually praising the showmakers for coming up with this ingenious way of writing an SoL that made me look forward to the next episode despite having a slow pace.

3) As far as the backstories of the characters are concerned, the showmakers basically go down the amnesia route where usually more details are revealed as time goes by. However in this particular show they just dont give much of a shit until the very end and only in the cases of reiki and rakka do they bother giving details of some significance. I would have preferred if they had actually made reiki the protag as she was more interesting and the details surrounding her life are intriguing to say the least. Also.

Spoiler, click to view

kyuu was not the main focus and hence her ascendance was something that I could not really give much of a shit about but the characters of the story did and they felt loss and grief.... but due to the aforementioned reason I could not sympathise a lot

4) Early on I was paying a lot of attention to the symbolism,

Spoiler, click to view

fallen angels, clock maker, baker , ruins , library lore etc

but as we reached the end I did not see it amount to anything so I settled with it being present just for le cool aesthetics. And thats fine too I guess. Some of the sequences in the second half were well executed.

Spoiler, click to view

Rakka getting rid of her sin bound status as well reiki doing the same with the help of rakka

were actually events that I will remember when I think about the show ,for the forseeable future.

Conclusion: The ambience fucking killed it for me, hands down the best part of the show. Reiki and Rakka were characters that complement each other very well. Pacing was slow but thats not necessarily a bad thing. The montage towards the end of the OP credits where all the main girls are shown together along with the music peaking at that particular moment, will be pretty memorable. Worldbuilding was lacking. Some characters could have been more well written if the anime producers had tried to stretch it a bit longer than 13 eps.

And overall it was a very good experience. 8-8.5/10 for me