Is it worth flying within Korea instead of taking the train or bus?

For Jeju, flying is absolutely worth it and is by far the main way people go, since there is no train and the alternative is a long ferry. The Seoul Gimpo to Jeju route is one of the busiest air routes in the world, with flights every few minutes, about an hour in the air, and often cheaper than you would expect. For destinations on the mainland, flying is usually not worth it over the KTX, which runs fast and frequently from city center to city center, but Jeju is the big exception.

To book cheap Jeju tickets, use the low-cost carriers Jeju Air, Jin Air, T'way, Air Busan, and Air Seoul, alongside full-service Korean Air and Asiana. The easiest way to compare is the Naver flight search, which lists all carriers side by side, or an app like Skyscanner, and the airline apps sometimes run their own promotions. Prices swing a lot, so flying midweek and at off-peak hours, early morning or late evening, is far cheaper than weekends and holidays when fares spike and seats vanish. A weekday Gimpo to Jeju fare booked early can be very low. Domestic flights are simple: bring your passport or ARC for ID, arrive about an hour ahead since security is quick, and there is no immigration since it is domestic. Booking a couple of weeks out for the best weekend fares is the main trick.