Originally Posted by jpatokal
Transferring between ICN and GMP isn't actually that bad; they're on the same side of the city and connected by a highway, so you don't need to go into central Seoul and can also avoid its nightmarish traffic jams. Buses leave every few minutes and the trip takes around 30 min.
Transferring between ICN and the Express Bus Terminal in Busan is not that bad either.
Benefits include no need to touch your luggage until Busan, far better views of the mountains and valleys than from the KTX and the unique chance to eat a Kimchi stew next to Korean farmers and truckers at one of the Soviet style highway stop complexes. If you are lucky you might just get to experience dusk or dawn against the wonderful mountain landscape. ^