If arriving by airplane from a foreign country you will more than likely be arriving at Incheon International airport. It is about a 90 minute drive into the city. Similar to the Air France buses in Paris Korean Airlines operates a number of "limousines" to the city which are reasonably priced, at about $15 (14,000Won).
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 3 across seating in business class style seats. They give free chilled 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. The cheapest hotel on the route is the Koreana Hotel, City Hall line.
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 the Gimpo domestic airport.
Taxis are broken out between regular and deluxe with 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%. Almost everyone takes a bus, due to the distance.
Also there are Seoul city buses that reach varying parts of the city for about $9. Take the KAL buses over the city buses especially if you are going to be staying at a major hotel such as Ritz Carlton, Hyatt, Koreana, Lotte, Westin, etc.
The standard limos are about 1/2 the price of the Korean Airlines limos, offer 4 across seating, no help with luggage, and will only stop NEARBY hotels. About $9. Sometimes you will have to cross the street for the return trip, which in Seoul can be as much as a 20 minute walk. So if you have lots of luggage, better to take the KAL deluxe limo.
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 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.
