Between season 1, season 2 and the OVAS, Kako-hen is my favourite. In a shoujo anime feelings are important and these 4 episodes didn't let me down.
The problem with time-travelling is that sometimes it almost looks like the creators are trying to seem smart. They try to build a web but that web ends up full of plot holes. This time-travelling little story on the contrary was simple and concise. And when I say simple it is in a good way.
There were some little things that weren't exactlly 100% explained but if we start thinking we can easily feel the gaps (and there are kind people everywhere to fill you up on information about the manga)
There was just one thing that I didn't understand. Maybe a plot hole? Maybe I'm just dumb? Nanami is still a God when she goes to the past so why Tomoe can't see the mark on her forehead? First I thought it was because he was not a famillier but I think in the first season there were Yokais that could detect the mark on her. She was basically a lighthouse for all kind of greedy beings. So how in the past she could easily pass for a human? Did I miss something?
Anyway, I really liked the pace of Kako-hen, as well as the overall reactions of the characters and the dialogues.
I think this is something that anyone who likes Kamisama Hajimashita can't miss. It's not your regular dispensable OVA, this is honestly very important, a big part of the story. It's not like the majority of the OVAS on series that are there to give you a good time but that don't really add to the plot.