I found this book to be a little duplicitous, in that I absolutely adored the first 60% and uncontrollably hated the second half. The first half focuses on the very strong and well-argued thesis that humans live by metaphor, and indeed that metaphor is our only means of understanding abstract, non-directly-experiential concepts. It's repetitive at times, but not distractingly so.
The second half, however, is where everything falls apart. The authors ironically fail to notice their mind-projection of human-understanding-as-metaphor to the external world, and spend the remainder of the book arguing that because humans experience the universe through metaphor that there can be no absolute truth in the universe. It's nice that Lakoff and Johnson are arguing against the contemporary philosophical stance on this, and, while they get some things right, they're significantly further off-course than the Bayesians on the same subject matter.
Perhaps more heinously, the authors spend a good deal of the second half of the book arguing with straw-man objectivists in an attempt to drive home their conclusions, at one point quoting philosophers as far back as Plato to argue their claims (despite the fact that one of their central arguments is that western philosophy is impossibly flawed as it stands). It's a terribly disappointing end to an otherwise fantastic book.
If I had the ability, I'd rate this 3.5 stars, but alas, I do not.