The battle scenes and the combat systems are so well-designed that it puts most other RPG-esque manga to shame. The interplay between skills, the combat roles that are more fluid that your typical "Vanguard/Midfield/Backfield" system, and the world-building of this manga are all next-level. The character designs are distinct without being overbearing and their development is handled delicately, like Theresa for example.
In conclusion: For a manga about a guy who has harem powers that activate when he's asleep, with the occasional dungeon run thrown in, this manga is pretty fuckin great.