GoogleMaps works in Korea, but the reality is that it gets confused often. Korean addresses, particularly in Seoul, are complicated, most taxi drivers will look at you like you are crazy if you show them an address in English, and many places have similar names when translated to English.
Made by Korea's Google equivalent, NAVER Maps works (mostly) in English, but is far more accurate than Google. It has better information about public transit, can read Korean addresses properly, and has better search results for Korea.
Download NAVER Map »