Thoughtful, intellectually honest, well-argued and in my opinion fairly convincing. The message is that liberals and conservatives (in the American sense) have different "moral matrices", i.e. value different aspects of human life and society differently. In one sentence: liberals are more concerned with care for individuals, while conservatives value the social fabric higher. Obviously, the argument in the book is more complex than this.
I have some reservations about the arguments concerning religion. The book's main argument is clearly focused on the situation in the US, but it does have considerable relevance to the rest of the world.
I am myself a liberal, according to the book's classification. But I have lately seen more clearly the points of "the other side", and we are in real need of a discussion. Haidt is hopeful that it may be possible. Even if Trump is evidence to the contrary, maybe our current epoch may eventually prove Haidt right.