Now I'm curious how it would feel under a kotatsu. (Yeah yeah. There's no kotatsu here innour country. No wonder. Ours is a tropical place)
It's pretty good. I tried it at a close friend's house in Japan. The heating element is stuck to the underside of the table and it's warm but not SUPER hot, so your legs won't get burned. And since there's a blanket on top of the table to keep the heat in, it's very warm and toasty. So comfortable that once you get in, you don't want to leave. And it saves on electricity since the wattage of the heating element isn't too high.
I may actually make one myself soon since a real Kotatsu is a bit expensive to import from Japan. There are a lot of hacks online that you can follow, if you're interested. :3