It synthesizes the past 30 years of serious historical research which revolutionized the presentation of the history of the American West by rescuing the experiences of groups who had been relatively ignored by standard interpretations. Indians, women, blacks, Latinos, Asians, workers are dealt with at length and with sympathy.
The research of anti-capitalist/neo-Marxist, anti-imperialist and pro-environmental historians is summarized and we can see the importance of the challenges they raise to old style historians.
The range of topics is impressive, and the writing is lively and intelligent. (I'd say this is suitable for the college junior/senior level.) The bibliography is amazingly up to date.
The reason why I don't give it a 5 is its lack of balance. At times the authors editorialize crudely--with dismissive judgements ("nonsense") and exclamation points galore to show us when we should boo or hiss.
Less empowered (victim) groups are too often treated as noble, and the majority as vile. This is the Achilles heel of a generation of historians who went into this field with strong orientations and sympathies.
But even more than the distaste for the majority groups, the biggest drawback is the relative lack of attention paid to them. I'm not saying, in an old fashioned way, that they should extol the "achievement" or mindlessly glorify the "Anglos" or capitalists. There is too much solid evidence here that the achievements were not 100% beneficial and that the white males could act and think in apalling ways. But they were the majority actors and this book can too often lose sight of that. At times it feels like the center is missing.
Still, it's an impressive, thought-provoking book. (The section on attempts by cowboys to unionize should be treasured by anybody who was ever spoon fed the Turner thesis.) But it probably should be the second book to give a neophyte, not the first.