Firstly, if you're thinking of a career in/studying geology, then this is a book you should certainly read, written by an obvious expert. For the layman, however, there are a few too many specific terms (jargon). Although it's clear that these are required, and the author does well to explain them first, it pushes the book into academia and lessens its accessability.
The author explains the evidence we've got concisely and is very careful not to guess. This makes most of the text quite dry, which you only really notice in the last chapter because it's there that he lets himself go a little and gets passionate about how humans may be causing the sixth mass extinction.
So, for the student: an excellent introduction to this very complex subject; for the layman, very informative but be prepared to concentrate.