Went through SEL/GMP twice (LAX-SEL-CJU and LAX-SEL) when I've been over there. Haven't made it to ICN yet though, as a pax. Was able to take a tour when it was still under construction about 2 years ago which was pretty cool. GMP was ok, but nothing spectacular, like was mentioned above, rather run-down.
On both trips went to CJU. Again, nice but not great. Loved Jeju Island though, very nice, except the weather crapped out on us at the end of the trip (one of those typhoon things...
Do hope I get to go back again, sometime.