You can use your credit/debit cards pretty much everywhere-from taxi to small restaurants, it`s more accepted than even in the States, IMO.

If you are taking a shuttle bus (6018) to D Cube, you can pay for it by credit card, however, on the back, only cash is accepted-keep that in mind. Metro will require cash as well. On Insa-dong sellers prefer cash and will give you a discount if you pay by cash (even without cash you can get a discount in KR).

The hotels usually can change your currency for a good rate, no worries. However, ATMs never worked for me (Wells Fargo).