Yes, Seoul is a wonderful place and worth a visit.
Korean food is delicious, and it`s fun especially to find little inexpensive places where some of the most delicious Korean specialties can be found.
The city is not beautiful in the sense that some other cities are, but it is a great place if you are open minded, and it does have its own unique beauty to it (Mountains in the city, classical Korean Architecture, small streets with trendy shops and cafes in some areas, etc..)
In order to decide if it is worth it for you, you should read a guidebook and decide if it is the right place for you.
I think it`s worth two or three days for anyone- especially when one has the opportunity to do it as a nice stopover. Don`t need to pay for extra airfare, and it breaks up a long trip.
Be aware, that the weather will obviously be very different from Thailand (especially in the winter. When Seoul is cold. Think Chicago)
Have a good trip!