best to town is indeed the bus (called "airport limousine", but is a bus). Relatively inexpensive (9000 Won one way), and quite a few routes. Important: Check with your hotel which route/bus number you should take, AND
- maybe most important - how to get from the bus stop to the hotel! I had instructions, but they were wrong. No offence, but nobody spoke any English, street signs - if at all existent - were of course in Korean only. It was quite tricky and I was lucky to accidentally hit the tourist info. The guy there gave me a map and showed me exactly where to go - very friendly!
Rail connection is currently under construction. If your hotel is close to a subway station, but not to a bus line, you can also take the bus (I think there is also a train/subway now) to Gimpo airport, and then from there the subway line 5 to town...
Taxi is expensive - depending on where in Seoul, around $100 one way!