Disclaimer: I rate manga relative to others, so 50 is average, 60 is a little above average, etc. My score does not mean that this is a bad manga- I quite enjoy it.

Heavenly Delusion is a post-apocalyptic story told from multiple perspectives. Tokio is one of a group of kids living within a sort of dystopian school, cut off from the outside world, who one day receives a message asking if he wants to go outside. Maru and Kiruko are two travelers going through the post-apocalyptic landscape outside, fending off bandits and man-eating monsters with a strange gun while in search of "Heaven". The story follows both groups as it slowly unspools the mystery of the world.
Heavenly Delusion is in a similar vein to other SoL/mystery stories such as Kowloon Generic Romance, being mostly character-focused and slowly revealing parts of the mystery. Combined with the mix of perspectives, with the series sometimes briefly following minor characters as well, the series is very capably able to flesh out the world and the mystery while making the characters interesting and having even throwaway characters play a role in worldbuilding. The use of multiple perspectives proves to work really well once the story gets past the exposition, and sets the series apart from other post-apocalyptic stories.

As far as characters go, Heavenly Delusion has a fairly strong cast. Maru and Kiruko, who are the main focus of the series, both feel childish while still having the grit to survive in a hostile world. Watching Maru grow a bit over time has been a highlight of the manga, and Kiruko slowly settling from the role of hired gun into a sort of guardian figure while still pursuing her own goals has been well done as well. The kids in the school have tied really well into the mystery, though they aren't particularly standout and there are some sexual scenes involving them early on in the series. Though the scenes later tie into the overall mystery and aren't just fanservice, they are present in the series, so heads up I suppose. Minor characters tend to fall into the typical post-apocalyptic tropes- the bandits, the scavengers, etc, but those living in towns feel full of life and lend a lot to the atmosphere.
The overall atmosphere and worldbuilding throughout Heavenly Delusion is absolutely stellar. The post-apocalyptic world feels lived-in, and the people have created interesting systems of survival and towns. New locations are quickly fleshed out, often through mostly SoL chapters, and overall there's almost as much to find in the atmosphere and settings as in the plot itself.

As for the plot, well, it does a great job slowly peeling back the curtain on a series of mysteries through following Maru/Kiruko and the kids from the school, while still going off on little tangents to explore the world more. The pacing is fine, as the story doesn't really start to tackle mysteries for a couple volumes and as such has time to flesh out the world and characters a bit before trying to introduce new aspects but then tosses little nuggets of information pretty frequently. There's not much I can say about the story without spoilers, but the reveals are interesting, don't feel out of place, and overall are quite well done. If you dislike a slower pace you may take issue with it, but the series has thus far struck a good balance between reveals and general worldbuilding, and I hope to see that continue. There are times where plot threads have been left open, which I hope will be addressed in the future, but that's my only real complaint with the pacing.
Recently, the series has been a bit scattered, but again I have hope that it'll right itself in the future.

The art is fine. There's a lot of white space sometimes and detail is hit or miss, but the art does a capable job and the times when it goes from pretty clean to more detailed tie neatly into the atmosphere. Backgrounds, the "Man-Eaters" and characters often have solid, visually interesting designs, and composition and paneling are solid, particularly for fight scenes- which are often abrupt exchanges rather than trying to be more than a simple brawl. The artstyle works really well to enhance the world, and the result is that settings and characters have a lot of impact.

Overall, Heavenly Delusion is a solid post-apocalyptic mystery. With great use of different perspectives, a fleshed-out world, and solid characters, it's a standout series. I haven't yet started watching the anime, so I can't say how the manga compares, but the manga stands strong on its own and is well worth the read.