As much as I would like to believe all my friends and sources online that says "Wifi is free and can be found almost everywhere in Korea", I must say, that is true to a very limited extent!
The only places which I had true stable wifi was in my guesthouses, and inside cafes. You can't get free wifi on the streets, neither could you access T-Zone, Olleh or any other wifi which are locked.
Rarely, you could get "iptime" which is unlocked at random places, but it could be weak or you could lose it the next moment when you move away from the location.
Even as I am on the KTX train, which promises "Free Wifi", I was forced out of it after 10 minutes of use. When I happily thought I could still access wifi on my phone, it logged me out in 10 minutes as well. So, I found out that the free wifi is only available for 10 minutes each time.
My friends who already applied for the Olleh prepaid card also says that the "promised" free wifi in subways are not stable as well.
So, I would like to say, don't expect too much for Korea's "free wifi" as you are unlikely going to be connected all the time as you travel around. If you are a social media addict like me, quickly get a prepaid card as soon as you can, and add on the 3G data plan for about 20,000 won+ for 2GB a month :)
And by the way, even in my dormitory room in school, I do not have wifi as well! How bad can it get :(
