Full Moon O Sagashite is REALLY good but you also kind of need to know what you are going in for otherwise it might disappoint if you are not expecting its direction.
The basic way to explain it combines the fluffiest elements of shoujo and magical girl stories with sad undertones and backstories for most of the character cast. It isn't like Madoka Magica at all and doesn't have horror or action elements, but instead has twists that are moderately sad or depressing.
This genre subversion or even fusion is executed very well especially on the visual level. Costumes and art are over the top, and the shinigamis' cute animal forms are effective in creating a younger-shoujo, vaguely magical girl atmosphere. The main character is full of unfounded happiness and cheer, and her rise to idoldom is perhaps too easy- but that's all intentional as it makes the twists and turns even more effective. Because each sad reveal strikes such discord against the cheery shojo atmosphere the author has worked hard to create.
That being said, some twists are more effective than others. The BEST twist by far, the one that will live in my memory, is the Eichi reveal. That reveal combined with the MC's backstory surrounding it, and the shinigami's vague recollections around the time of the reveal was just unbelievably good.
Izumi's backstory was fantastic as well. I loved that the author tied his backstory to seemingly innocuous clues in his outfit. Some of the twists in the first volume were executed perfectly as well.
Other twists aren't executed as well. Both main shinigamis' backstories were bs and were too convenient. BOTH had ill-explained connections to the MC's family that didn't need to be in the story, I would even say SHOULDN'T have been written that way. I was expecting a lot more, especially from the female shinigami whose personality seemed to hint at things in her backstory that didn't actually pan out.
I also thought the Jonathan twist was... weird? It seemed like something the editor should have shot down. It was kind of stupid.
Overall Full Moon is a fantastic, unique work that will leave an impression. Most of the twists are hands down amazing, and the ones that don't work can still kind of be forgiven (I just headcanon a different backstory for both of the shinigami lol). If you enjoy a twist on the normal shojo format or a work that is deeper than it initially appears I would definitely give this a read.