I always found the DC shells shift around too much when flying aeros, and have a tendency to restrict vision when craning your head backwards. Also, there seems to be a fundamental difference in design philosophy between the HGU 55 and the DC shell - the HGU has a large clear space in the shell around the base of the neck, whereas the DC has a hard clip-on ridge which extends downwards from the back of the shell towards the nape of the neck - which probably contributes to the problems looking backwards. For what they are, the DC shells are vastly overpriced, but that could be said of a lot of aviation headgear.