Ruta de la Plata
I have just found this thread and having travelled the route (loosely) this year I have some general comments. I did not do it all, just the section from Zamora to Merida, about 200 miles. I avoided some of the towns and cities I have visited previously so deviated as necessary. The road generally follows the dual carriageway so has little traffic. In my opinion, it is not enjoyable from a motorcycling point of view. Not many curves and relatively flat. If you just want to head north south on twisty roads I would travel to the east from perhaps Avila south, or to the west in Portugal. If you are interested in culture, the road is excellent.
Zamora is a very interesting city with lots of historic buildings, some walls, fortress etc. Salamanca is fabulous, famous for its architecture and golden stone buildings. It has a beautiful main square, every style of building but famous for its Plateresque and Churrigueresque style buildings, its university and is a world heritage site.
I have yet to stay in Placencia, but Caceres and Merida are both terrific places to visit (both are world heritage sites, so thats 3 in 170 miles), particularly Merida if you are interested in Roman stuff. There is lots to see (Theatre, amphitheatre, aqueducts, bridge, temple, circus) and really needs two days to do it justice. Off route to the east of Caceres is Trujillo an amazing small town and worth a visit (both of these towns benefited from wealthy Conquistadors returning from the new world). 40 miles south of Merida and just west of the route is Zafra, a very nice small town, it has a Parador, two nice squares and interesting buildings.
Hopefully someone soon will follow the route with some accuracy and give Richard some detailed feedback. Thanks Richard (and Simon) for your time.