the road riding here in Korea is amazing. There are literally endless miles of twisties, with some of the most beautiful scenery you can imagine. Mountain roads, coastline curvies, bays and lakes. Its really a paradise of riding. I am in the south where the weather is milder, so the roads are typically in great condition.
There are very few gravel or fire roads to explore though. Small land space combined with many people means nearly all roads are paved. Combined with most flat space being farmed and the majority of space being mountainous, there will be mostly paved curvy roads and steep hiking trails.
The drivers are very conscious of motorcyclists. There are some strange rules such as motorcycles are not allowed on the major freeways. But overall there are little to no rules in respect to motorcycling, as long as you're not an douche.
I might as well warn you, American soldiers get a very bad rap here. Many of it is deserved as well. Drunk 20's idiot meatheads stumbling around and trying to have sex or fight with everything that moves tends to not sit well with locals. Thus most expats and locals try and avoid the soldiers, and you will be refused in many bars/clubs etc. Don't be offended, its just the way it is. Tip - on the weekends try and diversify your friends, including hanging out with some Koreans. It will open up your world 100x fold.