I currently live in Chuncheon, the opposite end of Korea, and I love it. I was recently on tour down south, and I will be participating in a rando event in Jellonam-do in April. That particular area is full or farmland, smaller mountains, and lots of islands--perfect for riding. Koreans deservedly have a bad reputation for driving, but I have sadly become habituated to it by now (although it took a while). There is a pretty solid community of cyclists in most areas, and Koreans love to ride in/for clubs. Once you do make your way to Korea, I would suggest heading into a local shop that sells imported bikes (as they will be in the know with regards to local bike clubs) and give them a check. You will notice lots of bike shops selling cheaper Korean brands, but they are likely for kids/non-serious riders. Last bit of advice, there is a facebook group called Han River Riders...
Thanks for the feedback, and the warning. I don't know if I'll get the chance to ride when I'm there, but it's always good to have an idea of what I'm facing before I go.
Korean drivers are terrible...I supposed I could get used to it. I rode in L.A. for a while, but back home drivers seem much more predictable. I've been here a week and haven't rode much because it's cold, the drivers scare me, there's not much light after work, and I don't have an internalized map of the area. I'll have to get over it but damn, what a culture shock...as far as cycling is concerned. If you're reading this and thinking of bringing your bike to Korea, don't be scared off. It's all right, just different.
I live in Gwangju in Jeollanam-do. South Korean drivers are indeed crazy, but they're not hostile towards cyclists like American drivers are. People don't buzz you here to try to intimidate you. Actually I find the air pollution to be much worse than the drivers. However once you get out of the city it's fine. Commuting in heavy traffic for days in succession gives me a cough though. Pollution mask is in the mail. The countryside here is amazingly beautiful for cycling. I've never lived anywhere mountainous before though.