Travelled to Seoul last year (March, April 2015) and rented an Olleh Egg at ICN from KT.

We're a family of four and all of us were using the Egg for the duration. No complaints that I can recall; it was worth every penny, in my estimation. Just be sure to keep it charged.

http://roaming.kt.com/rental/eng/product/wibro.asp

Pick up and return at ICN. I arrived around 4:30-5:00 a.m. and had no problem renting it.