If arriving on an airplane from a foreign country you will more than likely be arriving at Incheon International Airport, located about one hour from Seoul. You can get into the city by bus, taxi, car service, or train. Most people take the airport limousine bus service. But the train is new and efficient, departing from an underground railway station that is well signposted from Incheon Airport's arrivals level and offers the advantage of no traffic delays. It also directly connects to Gimpo International Airport which serves most of the domestic flights as well as flights to and from some major Asian cities like Tokyo and Shanghai.

On the bus, it can take anywhere from 45 minutes to two hours to get into the city of Seoul, depending on traffic. Similar to the Air France buses in Paris Korean Airlines operates a number of "limousines" to the city which are reasonably priced, at about 14,000 won (US$15). There is also cheaper option of the standard bus costing 11,000 won (United States $12). There is no real difference besides the free bottled water and slightly better seats that the KAL bus service provides. Buses usually depart every 30 minutes to most destinations in the city. They also go to Incheon and cities in Gyeonggi Province as well as some major cities throughout Korea (though less frequently).

There is a separate limousine bus service to what used to be the KCAT (Korea City Air Terminal) but is now called "CALT" in Gangnam. This is non-stop. If you fly Korean Air or Asiana, you can check your baggage-and even emigrate- in town at the Terminal on your way home; BUT this is Gangnam, not Seoul "downtown".

The Korean Airlines limo buses are deluxe with business class style seats. They provide a free bottle of water and help with luggage.  The main advantage is that they stop at deluxe hotels, door to door, both going and coming from the airport. 

KAL 1 City Hall (stops at the major hotels in the area as well)

KAL 2 Namsan (stops at the major hotels in the area as well)

KAL 3 Gangnam (stops at the major hotels in the area as well)

KAL 4 Jamsil (stops at the major hotels in the area as well)

There is also a direct bus to Gimpo's domestic airport.

There are two types of taxis. Regular-white, grey-  and deluxe-black- with the regular being about a $80 ride from Incheon airport,  versus deluxe being about twice that. There is a surcharge for late night and early morning of 20%. There is a so-called International Taxi with English-speaking drivers, which will do a fixed price deal. 

Now, rail:

There is now a direct and fast train called AREX between Incheon International Airport and Seoul Station. These express trains take 43 minutes, generally running on a half hourly frequency and much cheaper commuter trains taking 53 minutes, with several departures each hour in between the expresses. Check: http://www.arex.or.kr/jsp/eng/main.jsp. An express train is a little cheaper than a deluxe limo bus, but the bus MAY take you direct to your hotel.  A word of warning: buses MAY be subject to traffic delays, something that doesn't affect the reliable trains. The commuter train is about 4,000 won while the express is about 12,000 won.  The express has two by two comfortable seating; the stopping all stations trains have longitudinal metro style padded seats.

The stopping trains serve interchanges such as Digital Media City that offer quick interchange to other subway lines, while at Seoul station by walking across the concourse it is possible to connect with Subway Lines 1 and 4 via a series of escalators. 

Save money by purchasing a T Money public transport smartcard from convenience stores at Incheon Airport (including one through the barriers at the KORAIL underground railway station: the attendant will let you through). This applies to the "commuter train" on the AREX line, not the Express.

FOR PASSENGERS IN TRANSIT IN SEOUL:

Lots to do.  If you have a few hours, there is the Air Garden Hotel, inside the terminal, (if transiting, no need to go through customs/immigration).  It's upstairs, and has reasonable rates, including some specials.  One was four hours for about $40 giving you a private room, private bathroom, where you can get some rest and a shower.  Nearby is a restaurant, that might even offer room service to the Air Garden, probably the same company.  They had an 8 hour rate, about $60. 

 If you have more time, go through customs and immigration, and take a free hotel shuttle to the new Airport Town, about 10 minutes away, where nice hotels are about $45 for eight hours.  I liked the Hotel June, which had a great clean and quiet room.  The new airport town has a Lotte Department store nearby with a cheap and big food court.  Nice place to stroll around for a few hours before your next flight.   There are even some beachfront hotels around Incheon area, in warm weather might be nice for a short stay. 

The airport also offers transit tours, ranging from $5 to a nearby casino, to about $30 for 1/2 and $60 for full day Seoul city tours, even a DMZ tour right from the airport.  There is a desk for this BEFORE exiting customs/immigration. They open at 8 am daily for tours departing at 9 am.  Naturally, for these tours, you leave the airport, and must exit customs/immigration. 

You can easily store your luggage at the airport if you want.  Also, there is a place near the Air Garden Transit hotel, upstairs, where you can take a shower for about $8. The towels they provide are pretty small. If you have your own soap/towel, you can use the shower facility for free.

The upstairs "restaurants" aren't really full restaurants. Better to go downstairs (this is AFTER going through customs for departing flights), to the Wally Restaurants, near the center of the airport terminal.  There are two, one on each side.  One is Asian food, one is Western.  they aren't labeled.  Prices were very reasonable, and food was great and fresh.  You order at the cashier, they give you a number for pick up a few minutes later.  Generous portions too. 

Upstairs, near the Air Garden hotel, is FREE INTERNET. Sometimes there is no waiting at all, other times you must wait.  There is also a pay internet place downstairs, near gate 23, which was expensive, but no waiting.   Also upstairs is an economy lounge, open to all passengers, where CNN was going, and it was nice there. 

There is a place somewhere, possibly in a beauty salon, where they have massages.