First, ondol sends heat all over the room. It uses conduction, radiation and convection at the same time. On the other hand, fireplaces or forced air heating systems only use convection, so they only heat the upper part of the air.
Finally, ondol is economical. It stores the heat in the stone track for a long time. For example, 'Chil-Bul Temple' in Jiri Mountain in the south of Korea has a highly-efficient ondol room called 'A-Ja Bang'. It was built about 1,000 years ago for the Buddhist monks' 100-day prayer during the winter. It is said that at that time, heating the room just once made the warmth stay for 100 days! How surprising!
Ondol is becoming global, changing and improving. Nowadays, over half of Europe, including Denmark, uses floor heating in new buildings. One of the first people to use ondol in western buildings was Frank Lloyd Wright, who was a famous American architect. He was fascinated by ondol. "How can a room be so warm without anything visible?" He brought ondol into many of his famous buildings like the 'Falling Water' and the 'Guggenheim Museum'.
I think ondol is a great invention. It doesn't heat up that quickly, but once it heats up, the warmth stays for a long time. I think ondol is similar to me. :-) I don't get along with friends so quickly, but once I open up, I become great friends with them for a long time. I hope a lot of people around the world would be able to enjoy the warmth of ondol.