Maybe, the best part of this anime is characters: Black Rabbit, Izayoi, Asuka, You and others — all of them have their unique features — "gifts", all of them have at least one funny trait per each, — in short, all of them are very well thought-out characters. 8/10.
Next in my review will be sound — in my opinion, the worst part of this anime. Of course, opening and ending can't be called terrible, moreover, they're suited to the mood of this show, but something like this I personally heard in "Kill me Baby" and "Ebiten", for example. 6/10.
Story, enjoyment and art are united in one intention: when in the final episodes story becomes totally eclectic with the elements of one German legend and some historical facts from the Middle-Ages European history, art becomes similar to the style of the 80's and due to that enjoyment becomes really impressive.
Overall — 7/10. I definitely won't rewatch this, but I didn't regret the time spent on "Mondaiji-tachi ga Isekai kara Kuru Sou Desu yo?".