Just returned from Seoul. We have T-Mobile Simple Choice, and found it worked well, at least for my wife and myself. Upon landing I got a couple of text messages:
Free T-Mobile Msg: Welcome to South Korea. Unlimited text incl with your global coverage. Talk $0.20/min. More info
http://t-mo.co/tc
Free T-Mobile Msg: Unlimited web included as part of your global coverage. To purchase high speed data please visit:
http://t-mo.co/4G-Data
It was a 3G connection, so I was glad that we also
rented an Olleh Egg for the duration. The Egg was 5,500 Won/day, and we used it for two weeks. It worked well, provided one keeps it charged.
Our son was unable to get his iPhone to roam in Seoul. Apparently he had not updated his carrier settings in some time (these are updated periodically when you connect your phone to a trusted computer running iTunes), so that may have been the cause.