Daniel Everett might not be the best writer, but he got a story to tell...
First of all: the book is split in two parts, which the author clearly separated and that's how we should evaluate them:
Part 1 is the story how Everett came to live with the Amazonian Piraha tribe (as a missionary) and a range of stories of what happened to him and his family there. His writing style is not the best but well readable and he manages to convey clear emotions and pictures. Sometimes annoying is that he occasionally falls into preaching about his favourite topic: language! But to keep with it: Part 1 is a good although not very structured description of Pirahan life, behaviour and most important thinking.
Part 2 is a relatively technical piece on the Piraha language. It seems the main goal of writing this book was that Everett wanted to disprove part of the Chomsky'an theory of language - as he tells us, living with the Piraha and learning their language, to translate the bible, made him loose his faith but he found a new love: linguistics. And this book, part 2, describes the Piraha language which, as it turns out, disproves Chomsky's theory (which is still the most influential in linguistics) in many respects. So, this part is rather boring and while part 1 feels like a real-life adventure book part 2 is often a bit confusing for anybody without a solid footing in linguistics or philosophy of language. It becomes clear that part 1 was not written because of the interesting story Everett wanted to tell, but to put his own theory of language into context: that language is environment- and culture-dependent and that what most linguists these days do - to sit in an office and analyse grammars - is the wrong approach to understanding language and culture.
Overall: The book is interesting in itself and many of the stories are truly enlightening as the reader is forced to reevaluate his own perception of culture and language, and to admit that many things we think are natural, even necessary are just cultural artifacts - and that maybe not having them would even make us happier! Everett describes the Piraha way of thinking and acting in an incredibly thought-provoking way, e.g. that nothing is believed for which no eyewitness is alive, that what I cannot perceive doesn't exist (and that what comes into my perception comes into existence!) and that there are spirits all around us.
The book is a great read, although I found the first part in every respect more valuable - the second part should be in a separate book for linguists. But, I guess, since Everett wanted exactly that not to happen, that people learn a language without understanding culture, it is only consistent that he kept them together.
Read it, enjoy and get an amazingly different viewpoint on life - and if its too hard just skip the second part and read only its last chapter, where it gets personal again.