I am a third year geologist at the University of Cambridge and this book is invaluable for the petrology part of my course. It is clear and well-written and an excellent complement to my lecture notes and the papers I have been reading. It is probable that you are looking at this book because you are either interested in geology or are an actual student, so in this case I would say that it is a worthwhile investment of your money. Despite stating that it is an introduction, it goes into sufficient detail to still be useful in the 3rd year of a tough course and I would definitely recommend it.
(Note: This review was partly written to give an honest viewpoint on the book and provide a useful review compared to the off-topic dishonest rating given which could have put off potential readers.)