I live in Seoul, but have done a bit of traveling around to other cities. I won't go into the working part of living here, since you've probably done a lot of research about it, and know what to expect. I'll just suggest that you be prepared for some unexpected things!
That aside, I really love Seoul. Before living here, I had grown up in suburbia and never lived in a one of the absolutely largest cities in the world. But it has been fun. There is a lot of excitement here, and of course if you dig around enough, there are lots of interesting things to do/get involved with. Transportation is great in Seoul, and really in any big city, but getting around efficiently will take all of your Korean skills you're learning here! If want to take a class of some kind, maybe graduate school..etc, or if you want to do lots of things in English, you'll find most opportunities in Seoul.
Living in a smaller town or (gasp!) even in the country has its benefits too. You will almost certainly improve in your Korean more quickly as you get further and further away from Seoul. That's both good and bad, because sometimes, you'll just really want to have an intelligent conversation in English.
You are never too far away from anything in Korea. Just about everything in Seoul can be found somewhere else... except maybe ethic food from other places in the world. Seoul has some great Indian and middle eastern restaurants! But Korea is small enough that going out of town in search of a festival or park or something is never too hard.
I've also spent a few days in 여수 and 목포 on the south-west coast. They are MUCH smaller than Seoul, and they have far fewer foreigners living there and passing through. In towns like that, you will be much more of an oddity. Again, that might be good and bad. You'll make Korean friends easily (and you can always find foreigners). But you might have to deal with people shouting "HELLO NICE TO MEET YOU" or "I LOVE YOU. SEE YOU AGAIN" as you're walking down the street. That almost never happens in Seoul.
Of course there are even smaller podunk towns, which can be really quiet. So think about what sort of social life you want to have.
Hope this helps! Let me know if you have any other questions! I assume you're coming here and planing to teach English?