Well, this post should go to travel - destinations - Korea.

Anyway, to answer your question, credit card is very well accepted everywhere include most taxi. Exceptions are airport bus (purchase ticket at ticket booth inside the terminal - they may accept cc), all the metros, and street foods. You may want to cash KRW a little bit at the airport because you'll need some despite super expensive fees in the airport. Then, go to any banks in in the D-cube city building in their business hour and exchange more KRW. Most banks in the city EXCEPT CITIBANK have very good rate and low fee for exchange. There's no "fee" for exchange anywhere, but the "rate" is different between the airport and downtown, and between business hours and outside those hours.

US dollar is NOT accepted anywhere beside several department stores which will charge extra fee.