Just did this a week ago. Unfortunately, the surface was nothing as advertised. Hope that changes soon, as the road has quite a potential.
With so few good roads in the North of Germany, our taxi-driver in Hamburg actually mentioned this road. And Hamburg is far away!
This stretch across the Harz in direction West-East is also
known as the Harzhochstrasse (Harz High Ridge Road).
As the name suggests the road runs at pretty high altitude basically on the ridge of the mountains.
It is a pretty wide road with super asphalt designed to carry a lot of trafic at high speeds. But there isn't a lot of trafic.... :o)
So it basically turns into a high-speed course with plenty of pretty high-speed, but still safe...!, action. I have several times witnessed bikers there at high speeds performing heroic knee-down action.
Ride safely - but have fun...! :o)