My Review is more of the Southern end of the route, from Rhayader to Llandloes, but I full agree with the OP that the whole road is a great little B road ride.
I have ridden the bottom end of the route over the years, but usually as a link when out trail riding. not really as a road ride in itself. Out leading a mate on some back roads of Mid Wales, and I thought as I planned the ride out of Rhyader a bit of ,"not again" on the A483 Crossgates to Newtown road. So routed up the B4518. What fun it was too....
Swoopy, blind tight bends, some faster bits... the lot. Super. Only thing will be farm traffic at busy times I suspect. Late September was fine.
Very scenic route through some impressive curves. Highly recomended.