I just spent 4 days in Seoul at the very beginning of March.
Yes, the weather was a little nippy compared to the Australian summer I had just left, but not freezing (highs of 10c/50f).
Personally I thought Seoul itself was basically just another city - not up to the excitement levels of say, Tokyo or Hong Kong - although many would say a more "real" experience.
The highlight for me was a visit to the DMZ at Panmunjeom, which I found fascinating, although you do need to be at least a little bit into politics and/or history.
Overall, I would say it is worth a few days as a stopover.