Out of all of the places I've driven around the world, I can safely say that driving on Jeju island is among the easiest.
The roads are well maintained, the traffic outside of the cities is minimal, the speed limits are appropriate albeit a bit slow at times, and with built-in GPS in most cars navigation is a breeze (although you'd be wise to confirm that last bit prior to renting as even Kakao Maps was fairly slow for us via our phones).
Based on this, I would highly recommend all visitors to get a car, and there really are only a few special considerations to keep in mind when visiting.