I spent a couple weeks in Korea though only 3 days in Seoul. Basically Seoul is the only part of Korea that gets any tourism so it's a lot cooler than the rest.
Pretty much anywhere I've been in the world there has been a lot of English being spoken but don't expect that in Korea. The cabbies and waitresses know squat so expect to do some pointing.
If you go you should check out the lonely planet forum (I don't know if the "no forum links" rule is still in effect, even though it's not adult related) and post a question in the Seoul forum about things to do.
One thing about Korea is that they are the Irish of Asia. Every restaurant will have a $1.50 bottle of Soju on the table! It's kinda known as cheap Sake and it takes a bit for it to grow on you but I'm hooked now. Oh and DO NOT drink 2 bottles of it, you will have the worst headache in the morning.
BTW if you're in Seoul check out the times for the USO tour to the DMZ
