They are very good. They series is the culmination of a massive research effort by the Militärgeschichtliches Forschungsamt and they manages to synthesize a great deal of material on the German side of the war. The reviews of the series in the academic presses have been universally positive. I myself use the various volumes here on AH as they are a fantastic reference. As with many anthologies, some authors' contributions are stronger than others and the military focus of the series does mean it gives shorter attention to some areas of the war experience. But the only real downside of the series is its cost. The books are definitely worth reading to understand Germany's perspective on the war.