I hope this review can be useful. English is not my nacional language so It will probably has some mistakes.
Story: It's completely focused on Megumin telling how she ended up in Axel, showing her daily life at school and at home. It's a good complement for the main story and explane more about the Crimson Demon and theirs caracteristics.
Art: It's great. The art remembers the anime style.
Characters: The main characters are Megumin and Yunyun. The two of them grown up a lot in this novel and the development of theirs friendship is great. The new characters are good as well.
Enjoyment: It was great. It's funny as a konosuba manga should be and sometimes sad. It was really a bad to see Megumin starving and being rejected by everyone in the guild (It's not spoilers, beacuse she told about it in the anime).
Overall: I'm a big fan of Konosuba. I've read all of Konosuba's light novels and mangas. So, I can say for sure that this Spin-off will be enjoyable for anyone who likes Konosuba. In my opinion, This manga is a good option for the ones who doesn't like to read novels.