For domestic Korea, I take a train as opposed to flying.

The Seoul to Busan train takes about 2 hours and 45 minutes. The flight is an hour but the commutes to and from Gimpo and Kimhae airport (versus train stations in the middle of Seoul and Busan), plus having to get to the airport early make the total travel times similar.

When they make the line between Daegu and Busan high speed rail, it should cut the total trip time to a little over 2 hours or so.