I currently live in JC. We really like it for the most part. I grew up in another part of East TN so I may already have some bias. I miss some of the bigger city luxuries like nicer/better restaurants and movie theaters, but JC is improving in those areas and Bristol just opened a brand new theater which is around 30 mins away. There is also a lot of new shopping opening in Bristol. Asheville is only an hour away and has great food and good shopping as well. Knoxville is about 1.5 hours away so not too far. We drive down a couple of times per month. The city schools in the area are supposed to be pretty good. The county schools not so much, but county schools in general seem to have a lesser reputation than city. (I went to a county school and think I turned out alright.) There are several hospitals around. JC is full of medical offices in part because the med school and VA. I probably haven't lived long enough to get a firm grasp on the variations in cost of living around the state or country, but JC seems to have a low cost of living in my experience.
I don't go to Kingsport too often, but Dobyns Bennett is probably one of the better high schools in the state. Of course Kingsport has Eastman which is the prominent business in the area. Kingsport doesn't seem to have too much to offer in the way of things to do, although I have heard good things about the parks. JC has more things for the younger crowd due to ETSU being in the middle of the city. It is definitely noticeable when summer comes because the roads and city are less crowded without so many students. The area has good outdoor recreation with parks and hiking just minutes away. Good places to ski in NC only an hour away. Unfortunately, the local lake in JC will be empty for a few years due to a sink hole near or under the damn, so that has caused a lot of people to put there homes for sale that are on the lake. But there are other lakes nearby such as Watauga which is very pretty.
Sorry for the sloppiness of the post but hope that helps!