Have done this before in a car and thoroughly enjoyed it, but this summer did it on the Le Mans on the way to Edinburgh and then Shetland for the Simmer Dim rally. It is even better on the bike.
Sparsely trafficked, parts are quite wild and you need to look out for deer, but the road surface is mostly OK and you can keep up a good pace. Great scenery and a good mix of different bends. I would strongly recommend as an alternative to the A702 that is more commonly used to get to Edinburgh from the SouthWest.
Of course the roads on Shetland are an entirely different thing from mainland, fantastic for bikes and some of the best riding I have done.