I was in South Korea last October and there is a bit of useful info: contactless payment is widespread, but mostly with physical card that you need to buy, recharge and throw away after visit (as for tourist). There is an alternative - one could use an app with pretty the same functionality: install (T-Money on Android), transfer money from your debit/credit card, switch on NFC and use everywhere you need it, be it metro, bus, convenience store or whatever.