The Promised Neverland, like many series, is one that started out strong and then ended in a rather unsatisfying way. However, it was pretty consistent for around three fourths of the story, so it's hard to say it's bad.

The main appeal of the series is the tense plot and exciting mind games, and while some parts do add variety, the roots stay the same. The characters are at their best when each is unique in personality and motive. Even to the end, the story reinvents itself and stays fresh, with only one caveat I will discuss later.

The first arc is the best. Every character has interesting motives and character, and in most ways its a standalone story one could read and move on from. The next arc is also pretty good, with new characters and an interesting world. It is more action heavy, but the strategic element of the beginning is still there.

Then we reach a problem. The next arc is setup for the finale, and it sets the story up to become a 10/10 masterpiece, honestly. The groundwork for an incredible moral exploration is there, but in the end, it doesn't follow through. I don't want to spoil anything, but the ending is just a bit too happy and convenient in my opinion, and the main character doesn't grow any in the final stretch.

One minor problem is that even halfway through, the amount of characters feels bloated. Now, most of the cast are children, and seeing them die would probably be upsetting to many, it would've been the best way to keep the cast fresh.

In terms of art, this is pretty good. Aside from the technical value, the art is simply unique in style, and it fits the series very well. The design of the 'demons' in particular is excellent.

Despite its problems, this is still a good series, and at the very least, you should read the first arc. The anime adaptation is also a great way to experience the story, although the second season seems to be heading in a worrisome path, so you may want to just read the manga after the first arc ends.