I did the west coast section - Inverness to just past Durness - last August.
Is it busy? It depends on the time of day. I did early starts so usually had a few hours to myself. But in the peak times - mid morning, early evening - there were quite a lot of camper vans, bearing in mind that it is mostly single track road with passing places, so you have to pull in to let them past. Mostly good natured: lots of waves and I can only recall one angry motorist (who had mountain bikes on the back).
I came back through the middle, via a version of the Great Highland Trail. After I turned off the NC500 route, I felt a real relief to have got away from the camper vans. On the inland roads, traffic was very light - 1 or 2 per hour on an A road. One passing motorist wound his window down and slowed down for a chat.
Next time I go back up there, I'll stay inland, for the solitude and the majestic views. The coast, as wonderful as it is, gets a bit samey after a couple of hundred miles and I found the traffic enough to be irritating.