As many reviewers here have said, the book is a sea of controversy, challenging a number of important generally-accepted arguments about Mao and his rule.
Some of the most prominent claims:
Mao's China was Stalin's creation; Mao had received money from the Soviet Union early in his career, and later served basically as Stalin's puppet (though a cunning one at that). Mao's rise to power would have been impossible without the Soviet backing after 1945.
Personally, I think the authors fail to come up with enough evidence to support this argument. On the whole the book seems to misrepresent the Soviet actions and motives. There is plenty of evidence in the materials, to which the authors had full access (Stalin-Mao correspondence, Mikoyan talks in 1949 etc) to at least raise serious questions about Stalin's real preferences in China; in fact, much evidence suggests that after 1945 Stalin was initially willing to trade in his special relationship with Mao for a broader agreement with the Guomindang. To this end, and probably not to confuse the "imperialists", he sponsored CCP-GMD peace talks, handed over territory to the GMD, agreed to suppress anti-GMD rebellion in Xinjiang and Altai, etc. The general tilt of Soviet foreign policy in 1945-46 - not only in China but elsewhere - points to Stalin's willingness to compromise with the West, rather than a propensity towards endless expansion of the Soviet sphere of influence.
A related question: the authors argue that the GMD collapse in the Civil War was a result of infiltration by communist spies and of Jiang Jieshi's kind-hearted treatment of the Soviet "moles" in his ranks. The authors then point to various Soviet "sleepers", in whose ranks they include people like Zhang Zhizhong, in my view downplaying internal reasons for GMD disunity. After all, one did not have to be a Soviet "sleeper" to want to play on the Jiang-Mao conflict to maximize personal benefits. Even if some of the so called "sleepers" in fact did, as the authors claim, had contacts with the Soviets before, this need not mean that they would act on Soviet behalf in China or carry out Stalin's orders.
Another major pillar of the book - the role of "ideology" in Mao's rule. The authors emphasize that Mao was moved first and foremost by considerations of power and prestige, not by fears about "revisionism" in China, etc. I happen to completely agree with the authors on this point; their argument, backed by detailed evidence, goes a long way to remedy a certain disbalance in favor of the "ideological" school especially popular since Chen Jian's path-breaking Mao's China and the Cold War.
Portrayal of the Cultural Revolution as basically a power struggle is fairly convincing, though one cannot help but have second thoughts. Other authors, including notably Roderick MacFarquhar, admit that there was indeed a power struggle, and yet could there be something else, or why would Mao continue the campaign beyond 1966? In their effort to portray Mao as a cynical manipulator, do the authors overlook his genuine concerns about China's future path?
An interesting thesis comes towards the end: Sino-US opening was not a result of Mao's fear of Soviet invasion, but of his effort to regain international prestige after the failure of Maoization of the world. This is a very important argument for what it tells us about Mao's delusion of grandeur, though it probably downplays Mao's fear of the Soviet Union. Yang Kuisong in a well-known article in 2000 issue of Cold War History journal argued that it was precisely Mao's reaction to Zhenbaodao in 1969 that prompted him to rethink relations with the US. This claim has been advanced in other books, for example, Li Zhisui's Private Life of Chairman Mao.
Speaking of the Soviet angle, the authors advance a controversial claim that Malinovsky's drunken remark to Zhou Enlai and He Long in November 1964 about ousting Mao was actually a carefully planned act of official policy. There is no evidence for this. In fact, drunken orgies were a common practice in the Soviet leadership ranks, so Malinovsky could well say rash things without any instructions. Indeed, if the Soviet leadership wanted to probe Zhou and He Long about removing Mao, why not do so privately, rather than in the presence of foreign diplomats, including those from Western countries. Furthermore, it was He Long who approached Malinovsky to make remarks, and not the other way around.
Of course, there are many episodes that are still impossible to document properly. The verdict is out, but this book gives interesting interpretations.
Regarding Lin Biao, the authors subscribe to the well-known version that the plane crashed in Mongolia because it ran out of fuel. This is one example of mysteries-still-to-be-solved. Indeed, a recently leaked document (investigation by the Mongolian KGB) concludes that the plane had plenty of fuel on board. This evidence was not available to the authors at the time of writing; it shows though that many of the arguments advanced in the book will be challenged and possibly overturned when new evidence emerges.
Many reviewers have said that the book blames everything on Mao. That is true to some extent. Clearly, the authors emphasize the role of one man over the faults of the system. However, I feel the authors take a fairly bold line in condemning Zhou Enlai, whose "myth" is perhaps more wide-spead than the myth of Mao. Zhou, Liu Shaoqi and many other cronies willingly participated in Mao's purges. For some reason, Peng Dehuai is spared criticism in the book, as if he was the only saint in the leadership. Deng Xiaoping receives light treatment, too, though one might ask where he was in the late-1950s at the height of the economic mess? Did he not also advance Mao's foreign policy program rigidly, for example in Moscow in July 1963, where he was more outspoken against "revisionism" than even Kang Sheng?
The amount of work that went into this book is simply colossal. It must be the best-researched book on Mao ever written, and it does offer a great deal of new scraps of evidence, though some of it comes from highly questionable sources such as "interviews with a Russian insider" (???). It is impressive how the authors were able to memorize (!) documents in the Chinese archives and then quote from them. The level of interviewees (presidents of different countries etc.) makes me wonder how the authors pulled this off. Overall, I am very impressed by this book. It is exceptionally well written.
Are the arguments too far-fetched? It is hard to say. Almost every major and many minor claims of the book could be easily challenged as I showed above. And yet... The final words somehow ring true: "His [Mao's] mind remained lucid to the end, and in it stirred just one thought: himself and his power" (p. 616).
I walked away with a mixed feeling about this book, but it sure made an impression.