After hearing Ron Walker conduct an hour-long Q&A on the book last month, I quickly finished Massacre at Mountain Meadows a couple of weeks ago. The narrative moves quickly, first laying out the historical context of the Utah War and the events in Southern Utah leading up to the massacre, then focusing on the grisly few days at Mountain Meadows. The authors reject the idea that the emigrants incited the attack and also clear Brigham Young of any direct role, placing responsibility squarlely on the locals, especially John D. Lee.
My primary criticism is that the 231-page narrative is too short. Given how controversial MMM has been, I think the authors should have addressed alternative interpretations of the controversial issues more directly, either in the text or in the notes. Bushman did this very effectively in his Joseph Smith biography, Rough Stone Rolling. I know the response is that the authors (or the publisher) didn't want 500 pages of text with another 200 pages of notes, but I really expected more discussion of the disputed points, not just a new and improved narrative.
FAIR is posting some nice analysis of the book: Part 1 includes discussion of John D. Lee, described as an abusive and power hungry zealot. Part 2 is a discussion of Brigham Young's Indian policy, which was covered in enlightening detail in the book and was, I think, one of the stronger treatments in the book.