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Except for spanning across a huge area which reaches all the way to e.g. Incheon and Suwon, the subway is also easy to use for visitors who do not speak Korean. Every station has most of its essential instructions and directions written in English as well as Korean. Furthermore, the subway net is easy to use with only a few minor traps which might cause confusion for the passenger. Two examples are the rapid trains which pass certain stations, so one should be mindful when boarding those trains to check if it actually stops at the desired station. Furthermore, some stations such as Noryangjin requires the passenger to actually exit the station when transfering (but still doesn't charge you extra! I still do not know how this is detected on the card when charging ^^).
For the pros, we all know it is comparably cheaper compared to other metropolises and also very clean if you consider the hordes of people who actually ride. The upsides of the subway system are numerous but I think I will settle for this picture which tells it all.