I'm one of those few people who read Angel Beast!: Heaven's Door prior to watching the anime. However, despite the fact that I haven't watched a single episode of the anime, I really enjoyed this manga for a lot of reasons which I'll discuss below.
Story (6)
As I said earlier, Angel Beats!: Heaven's Door is a prequel to the original anime. It focuses on the events that occurred before the original timeline. The premise of the manga is very simple, and it takes a rather light-hearted approach in narrating story, especially when compared to the original light novel.
The story of Heaven's Door basically narrates the daily lives of SSS and its members. For the most part in the manga, the characters do silly, wacky, and occasionally clever things in order to figure out the mystery behind the world that they are living in. While there are some dull moments here and there, the activities of SSS are generally quite entertaining to watch. The narrative isn't entirely light-hearted, though. Fortunately, there's mostly a proper balance between the light-hearted and the rather dark themes. The backstories of some of the SSS members are simultaneously very interesting and heartbreaking, which makes the narrative more compelling.
One major issue that can't be overlooked, however, is how little information there is regarding the world that the characters live in. While this matter is probably resolved in the sequel, considering the fact the afterworld is a very component part of the story, the author should have explored it a bit more and given it more exposure.
Art (8)
The art is mostly very decent. All the characters are drawn in a decent and attractive way. The background designs are also fairly detailed and nicely drawn. The only gripe I have with the art is that some of the characters look kinda similar to each other or have similar character designs, which sometimes makes it hard to distinguish between them just by looking. Besides that, I didn't find any other major issues with the art.
Character (8)
All the characters of Heaven's Door are charming and enjoyable in their own way. They all have different personalities from each other and the interactions between them are mostly very entertaining. Most characters are given interesting backstories that make them more relatable to the readers. They are also given proper character development throughout the entire manga.
Since the manga is a prequel to the original anime, there are some additional characters in it that the anime doesn't have (such as Takamatsu's friend Mitsumura). But for this exact reason, some characters that appear in the anime don't appear in the manga (such as Yui).
As I said before, most of the characters are really likeable. But my favorite characters are the first two characters introduced in the manga, Yuri (or Yurippe) and Hinata. They are surprisingly a very entertaining duo despite their differences. Their character development throughout the entire manga is also very notable.
Kanade (or Tenshi) is a very likeable character as well but she doesn't appear much in the manga. Her backstory also remains very mysterious, which adds to her appeal. I am assuming her backstory is revealed in the anime and/or in the sequel of this manga, which would definitely give her a lot more depth.
Although I'm fond of almost all the characters, I did have a few problems with some of them as they don't receive much character development and appear kind of bland. A couple of examples would be Noda and Ooyama. Although these characters are probably flourished in the sequel, since they are one of the staple characters of the story, it definitely would've been better if the author focused more on developing them. But since it's hard to develop all the characters when there are a lot of them, it wasn't that big a flaw to me.
Enjoyment & Overall (8)
Angel Beats!: Heaven's Door is a manga that I would recommend reading to almost everyone, especially if you are a fan of the original anime. I personally enjoyed it a lot and have become inclined to watch the original anime. There are a few flaws here and there, but they didn't affect my enjoyment that much.
Overall, Angel's Beats!: Heaven's Door deserves a solid 8/10.
