I spend more time here than I'd care to admit, but believe it or not, Inchon Airport was rated by Time mag readers in 2006 as one of the best in Asia. This is a brand new airport with state of the art amenities, Cool designs, good lighting and lots of room (reminds me of a huge hanger). It appears to be a lot less congested and hectic when compared to BKK or Manila, and is not a bad place for a long layover.

Here are some tips:

. The airport is about 1 hour (taxi) or 1.5 hours by bus into downtown Seoul. You can get buses outside the baggage claim going all over to parts of Seoul.
. Many domestic flights still fly out of Kimpo, about an hour away by taxi.
. Asiana and Korean Air use this airport as a hub. Asiana, as you may know, flies directly into Clarke, Phils. UA has daily flights to/from Inchon into Tokyo and a non-stop to SFO.

. You can rent cell phones near the baggage carousels.
. Customs and immigration are usually efficient and fast (entry and exit)
. There is a free transit lounge with free internet stalls. Ask for the "transit lounge". They even have muslim prayer rooms, massages, and private lounges with showers.
. There are a few nice transit hotel where you can pay by the hour or day for long layovers. Usually located near the transit lounges.
. At each wing, there are western food plazas vs korean food plazas
. Many airlines, when you check in, are very efficient. Some airlines, however, will xray your bags so after you get your boarding passes, you need to wait a few minutes to make sure your bags clear.
. You must have a departure card filled out and pay an airport tax prior to entering the immigration clearance, they have machines that'll spit out receipt for your taxes which you need to keep with you.

. Inchon is about 1.5 hours to Tokyo, 1 hour Shanghai, 4 hours from Manila/Clarke, 6-7 hours to Singapore/Jakarta, 5+ hours to Bangkok.

Here's a good link for more info,

http://www.airport.or.kr/eng/airport/

or
english.tour2korea.com/01TripPlanner/Transportation/air_incheon.asp?kosm=m1_4&konum=1 - 50k