Just returned from Seoul, and I used an Olleh Egg plus my T-Mobile Simple Choice plan.
I was able to get the Egg for 5,500 Won/day for 14 days. I'm still confused regarding how that worked out;
the KT page quotes the 8,000 Won/day rate, and describes a discount for long-term rental which I won't pretend to understand. In any case, we found the Egg worked pretty well for us.
I did make a reservation for the Egg, but I'm not sure it delivered any benefit. They never emailed me a confirmation or quoted me a price, and though I kept a PDF of the submission, I forgot to email it to myself, however, so it was back home on my desktop HDD when I needed it. When I picked up the Egg at ICN, I filled out paperwork and was quoted the 5,500 Won/day price without the benefit of the reservation. It was only after I had completed the transaction that the clerk indicated she'd found my reservation, and it apparently changed nothing.
Regarding the T-Mobile roaming, as others have indicated, I found it worked well for me. My son had not updated his iPhone's carrier settings, so we believe that's why he wasn't able to roam there.
Upon landing, I got a text message which read:
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