Internet services in Korea is one of the best in the world. As long as you know use a more reputable telco services, you can get very good connections even if you are tucked away in some of the more secluded areas such as mountains or countryside regions.
The question is what package do you purchase? Should you purchase it at your local airport, or at Incheon Airport?
Here are some information and recommended option- meaning that I have found that it suits my needs better, and I am satisfied with what I get in return..
Recommended Option* :
Purchase a unlimited data only SIM card online at discounted price and collect at Korea's Airport on arrival.
Costs ; Product options; Quality. Don't purchase your data only SIM at official Korea Tourism sites. You can get better prices at authorized dealer such as TRAZY. You have the option to purchase a 5, 10 or 30 day unlimited data use at USD22.50, 31.50, 58.50. Your activation starts when you collect your SIM card. The SIM card sold here is by KT(Olleh), which is Korea's largest telco operator. To browse the telco options in Trazy, click on the banner link on the right, select your country> Transport and Wifi> Wifi and Sim cards. I am recommending TRAZY for Telco-Sim purchases, as I do find them quite responsive. The telco network had some problems on the day before I depart for Korea. I received an email from TRAZY informing me of the options in event that the problem was not rectified when I arrive. That was really a peace of mind and says something about their customer services and operatios. Fortunately for me, it was resolved when I arrived. That says something about the Olleh's services and operations.
Personal Review of SIM card/Connection: I read that some phones from some countries may need some setting adjustments to the phone, so I brought 2 phones, my primary phone which I use daily and an old 4G phone to hold the Korea SIM card and to provide tethering on the go. You don't need to do this- I just want to avoid the hassle of making adjustments to my primary phone due to change in SIM card. ( If you are from Singapore, there is no need to make adjustments, and no worry, the staff at collection counter will check it for you).
I was really impressed with the strong signal. To illustrate, my travel companion was in the next hotel room, and she can get a strong signal from the tethering, enough to make clear internet calls and watch online videos seamlessly. And we are at mountains and seaside or most part of the journey.
With the unlimited data plan, I really did not have to worry how many people are connected to my device, or if my own handphone had to update softwares etc. This was a primary complaint from products which offers fixed data plan. I also indulge in using the many travel apps for moving around Korea, which really goes a long way in making the trip smooth and truly free and easy.
The only thing you might want to take note (which is not a fault of the operator) is that tethering does consume the battery much faster than your normal usage. So make sure you bring along a spare battery pack. The other option to save on the battery is to use bluetooth tethering. I did not try it in Korea, but I have tried it in other countries. It seemed to use less battery, but I think the range of connection is limited by bluetooth technology.