If you are wanting remoteness and to get away from it all – in a long distance walk – then the Southern Upland Way fits the bill perfectly. On most days I did not see another person out walking, though by reading some of the visitors books in the bothies there is a regular trickle of people using the route.
If you are wanting a more sociable time then either chose a different National Trail, or complete the Southern Upland Way going east to west – and you might well meet one or two people each day walking the other way.
While there is quite a bit of plantation forest walking, and some road walking (but on very quiet lanes), the Southern Upland Way takes in some wonderful scenery with magnificent views. There are some long challenging days of walking, which might well deter many people, though with vehicle support or using some of the bothies the route can easily be broken down into manageable chunks.
If I had been sensible I would have planned to use the numerous bothies (there are 5) to split up some of the longer days. Undertaking the whole route in 11 days was probably too little time, spending a couple more days completing it would have made it much more manageable.
The route is very well signposted, the best I have walked by far, and there are just enough people walking the route to define a reasonably clear line in the grass ahead (though thankfully not too many that the path gets churned up or any boggy bit become a quagmire).
The remnants of the Waymerks Project (warning spoilers) added another dimension to the long days of walking, and the various art installations along the route, while not all to my taste, did provide bold statements. The most challenging items however were the various standing stones, cairns and memorials to darker times in Scottish history, not only did these provide a haunting connection with the past they helped describe a very different landscape.
I would highly recommend the Southern Upland Way, and if you have already walked some of the more popular long distance walks/trails I suggest you add it to your list soonest.map in Google Maps, or here is the Google Earth KML file.
More photos (from all the days of walking) can be found in the photo gallery.