I just finished marathoning the series an I gotta say I really enjoyed the first half of this series especially with Sheena. The second half was okay but no where near as interesting as before and felt very mundane and irrelevant like they were just trying to find a way to reach the beginning of Angel Beats
Seeing how Yuri built the SSS was very interesting.
I do find it weird how attached Hinata was to Yuri as that was nowhere to be seen in the anime and heck in the second half of this series it stopped happening.
I do like that the series shows that NPC are actually quite different and do have a personality of sorts.
I am disappointed however with this series not explaining peoples backstories and instead just hinting at it. A prequel felt like a great way to give all the characters backstories.
Still, I really enjoyed the series but I am still waiting for that Angel Beat series which explains peoples backstories (though the Angel Beat visual novel does that but there has only been one so far)