The book sets a good basis for comparative sociology of power and legitimacy. It observes two dictators who at least these days seem to be the most despised leaders of the 20th century. Lewin and Kershaw have brought together a handful of WW II top scholars, whose articles jointly create a sturdy basis for understanding differences and similarities between the rules of Stalin and Hitler.
This brings me to an important point worked on in the introduction: Scholars of history need not keep their hands off comparisons. Actually, no historian can completely avoid comparing phenomena, so it is better to be explicit about the whole thing than hide it behind the illusionary curtain of pure historical description. Stalin and Hitler are often compared in literature, but almost never does this comparison rest on a strong theoretical foundation. It is just so easy to bunch up two non-western autocrats and say that they are similar, because they caused millions of deaths, or because they had many fanatic followers, and leave it there.
For me, the most interesting parts of the book were the chapters about leadership cults of Stalin and Hitler. For example Lewin, Mommsen and Kershaw show us how the leaders built their structures of authority. Hitler's system of power was doomed to fail. It was self-destructive. Stalin, on the other hand, was able and willing to channel part of his divine glamour for the benefit of state bureaucracy. However conflicting wishes of the bureaucrats and the paranoid will of Stalin may have often been, Stalin's death did not leave the whole Soviet system hanging in the void.
Summary: Good overview of the subject, lots of intriguing visions.