My impression on this book from what people around told me before actually reading it was that this book is the canonical textbook for those who want to get into Bayesian statistics. After having read this book from cover to cover, however, I do not think it is a good idea to start learning Bayesian statistics with this book, as it covers very wide range of topics and therefore does not get into much technical depth for most of them. I think this book is ideal for someone like me who has very basic understanding of Bayesian statistics but would like to get some exposure to a variety of existing tools in the literature so that when some of them become needed at certain point of my career, I can reopen this book and follow its references to learn enough to actually use them.
The stance of this book is very practical, and it is great to get a glimpse of how these grand-master Statisticians approach data analysis. First few chapters regarding the underlying philosophy of Bayesian statistics is also very insightful.
I was somewhat disappointed with changes in the third edition though. The addition of Gaussian Processes and other advanced topics is conspicuously advertised here and there, but I found these new chapters to be relatively poorly written compared to those from the previous edition; notations are not consistent with previous chapters, and clarity of writing is disappeared. It was a stupid idea to buy the new edition while having the second edition.