We have... well had... a campaign going in Bonn. It was quite nice, but fell apart because out characters were messed up... (They were our first tries for the DFRPG...)
Bonn is not really big, but that's not really an issue. We had some very nice locations and Faces. And there are lots of Tales and Legends around Bonn.
Drachenfels, Siebengebirge (Dwarfs anyone ?), Father Rhein and his Daughters, Der Ring des Nibelungen (Ring of the Nibelung) and many more.
If you want to stat up the Ruhrgebiet... simply start with one city and add more as the campaign progresses. You'll be doing that anyways, since you don't want to repeat stuff all the time. You don't need a fully fleshed out Ruhrgebiet to start with, just some key Faces for the PCs to interact with... the rest is background.