Story: It's in a school. It's mini-episodic. All events are high school oriented and absolutely unoriginal. 1/10.
Well they use the characters in their proper roles I'm guessing, so 2/10. Still so lazy the effort is non-existent.
Art: The chibi works fairly well. Animation is very minimal and lazy too, but hey, as long as it works when it has to (I could watch Aqua being punched by Kazuma probably 10 times in a row)...7/10
Sound: Talented VAs from good shows, and a cute opening. It's overall very solid. 8/10
Character: Well as expected, it's always brutally dumbed down for these crossovers. If Persona Q failed at not dumbing down everyone, then I had, rightfully, no hope for Isekai Quartet, and well it brings down the characters almost to their most basic tropes. They did Tanya and Ainz fairly well IMO. I haven't seen Re:Zero, so I don't know about Subaru. And as for the Konosuba team, troping them isn't really a mistake since they are giant tropes anyway.
It's dumbed down, but it could've been worse, is what I mean. Still, 4/10, no miracle.
Enjoyment: 7/10 I thoroughly enjoy every time the Konosuba crew does something Konosuba, and even though I think Overlord is mostly underused, Tanya is pretty good. The relationships are written well enough to be credible and the jokes are far from bad.
Overall: 5/10: No miracle anywhere here. It's another dumbed down, low-animation, no-plot no ambition cash grab. But for a fix of your favorite Isekai characters, it'll give you your fix.
