This one is amazing. Even before I picked up "Isekai Office Worker" (that's the titel under which it aired where I'm from) I was wondering what one could do with Isekai and Boyslove combined. I'm in kind of a hate- love- relationship with both of them because most of the time they fail to fullfill my expactations and come with a lot of tropes, which makes stuff ridiculous, superficial and predictable. When I picked up "Isekai Office Worker" I had high expactations and it didn't let me down. While reading I constantly felt how much effort they put into this manga. The setting is an medievalalternate world. What differs to the average Isekai world building is e.g. the lack of a concrete enemy like a Demon Lord, which is refreshing. It's even smart because that way the plot really focuses onto Seiichiro and his surroundings.
The plot is worked out flawless - yes, it has an actual plot besides the MC's getting closer to each other. The characters are well done as well. They are consistent, come with their own strengths and flaws and throughout their story the ones the plot focuses on get character developement that feels natural.
I don't want to spoil too much about the male leads. Their flaws and strenghts work well together, so their relationship grows steadily without diving into tropes or iconsistency. The "BL spice level" is low but the romance is there nonetheless.

To sum it up, I'd recommend this manga to starters and veterans of the BL theme, even to those who dislike Isekai.