I've headed up to Seoul over the Chuseok holidays for the past 2 years (This time I'm getting out to Thailand) and at most I can say it is.....okay.
The city all but empties out and while you will have enough open to probably stay occupied and comfortable, many many things will be closed. The lively vibe of the city will be very much lost and you will wait longer for taxis and buses as they will run more infrequently and taxis will be scarce on the actual Chuseok days. I guess if you want a relaxing time in a big city (talk about a paradoxial situation) you can do it, but it definitely won't have the vibrance of a normal visit.

For extra reference, here is the website of the Korea tourism association that sets out the activities available and running during Chuseok. If any of these strike your fancy, then go for it!
http://english.visitkorea.or.kr/enu/...sp?cid=1723192