South Korea itinerary: Is Jeju Island worth the flight?

We have 14 days in South Korea next spring.

Doing Seoul and Busan FOR SURE, but wondering if we should take the domestic flight down to Jeju Island for 3 days.
It looks beautiful but I’ve heard it’s mostly just a honeymoon spot for locals and you need to rent a car to do anything. Worth the hassle or just spend more time in Seoul?