Did this route on new years eve 2010. Early morning. Completely deserted. Lots of lovely bends around the mountains and through the valleys. Not a fast route through Spain due to the many bends many of which are quite low speed. But amazing scenery. Well worth a detour for this.
A really good selection of tight hairpins, fast sweepers and a nice view as the road rises up from sea level to around 1000m. As this is rural Spain, you may need to pay some attention to farm animals straying into the road (although I didn't see any on the way). This route is ideal if you've just got off the ferry from Santander.
I think this is the highest mountain pass in the area, just over 1,900m and it gets quite cold at top, nice views when its not misty.
The road surface is good & wide, and as you can imagine with a mountain pass lots of corners. Was quite wet when i went along here in late May 08 but still worth it for scenery.
Places of interest on the way is (not sure of name, in English is something like Valley of the Dead) Church where Franco is burried (bit of a controversial place, in Spain, as Franco built it using soldiers captured during the civil war, a lot of whom died making the place) then onto El Escorial, burrial place of all Spanish royalty, impressive building.
Great scenery that changes along the entire route. The roads are mainly straight, the epic ones that go to the horizon in a heat haze. There are some wonderful views along the way with plenty of great little towns and villages that you go through which are a delightful. There are also a number of old ruins along the way which can be seen from the road. This isn't a riders road, but you'll be happy to take a relaxing ride and just enjoy sights and smells a long the way.
A good link road to use that's better than the A470. Recently resurfaced, with mostly fast bends. Brief but quiet enjoyable.