For those who don't know, the original VN for Dies Irae is roughly 70 hours long. They packed 70 hours of VN into 18 episodes, so what, 6 hours? In the original you have loads of time to get familiar with each member of the cast, their backstory, their abilities, relationships, etc. In the anime, you get so little time to learn anything about anyone that it's hard to care. Not to mention it was originally 4/4.5 routes, cut down to one in the anime. So you miss a lot of buildup or it gets insanely rushed.
In addition, the anime starts with scenes you don't even see till near the very end of the VN! Spoiling the answer to mysteries you've not even encountered yet... While in the VN the antagonists are for a long period of time very mysterious, the anime shows you so much about them right at the start before you even meet the protagonist that it is ridiculous. And even though it tells you this stuff, to someone who isn't familiar with the VN it would likely just be completely confusing.
And it jumps around jarringly sometimes that its so hard to keep track of what's going on unless you've already played the VN, it must be a terrible experience for people new to the series.
Additionally, in a series that largely centers around personal combat, they decided to add giant (as in like, 100+ stories tall or something) skeletons that some characters fight using, which was not in the VN at all.
I could keep going on, but instead, I'll talk about the upsides. For one, in the blu-ray version it has audio commentary by many members of the cast, which is quite cool. To hear them joking around and such. Secondly, I was totally surprised (in a good way) to see that it depicted a hugely important scene (for a moment), that had never been shown in any entry to my knowledge, just implied. It was super cool to see, as a Shinza series (the series name) fan. What it was, was (Pretty large Dies Irae, Paradise Lost, and a yet to be released Shinza entry spoiler):
Spoiler
Satanael's death at the hands of Mercurius.
Other than that, another perk is the blu-ray version has some pretty good fanart interspersed between episodes that I'd never seen before.
I've got 3 episodes left. Overall, I wouldn't recommend it to anyone but hardcore Shinza fans who'd already read PL and DI, and then I'd just recommend watching the commentary, and the scene I mentioned in the spoiler.
1
