Florian, I have most of the set here at my library (Wichita Public Library in Wichita, Kansas). We do not have volumes 6 to 11, or 13 to 15. I will be purchasing the new volumes when they become available, and look forward to the new one since it covers Kansas. I do look-ups all the time, so feel free to ask.
We also have several other useful sets on Germany:
The Atlantic Bridge to Germany Volumes 1 to 10
Germans to America: Lists of Passengers Arriving at U.S. Ports, 1850-1897, Volumes 1 to 67
Germans to America Series II, 1840-1849, Volumes 1 to 7
Map Guide to German Parish Registers Volumes 1 to 60
The Wuerttemberg Emigration Index Volumes 1 to 5
We also have over 100 Yizkor (Memorial) books from JewishGen for towns across Eastern Europe
German Immigrants in American Church Records Volumes 1 to 5, Volume 12, Volumes 16 to 37