It is hard to review or even to critique good books, especially this one. Simply say "great" or "must have" does not help potential readers decide whether they need to invest in this volume. I have been struggling as well due to the fact that this book costs money (considering that I am still a poor student) and time (for reading). I was unable to finalize my choice untill this afternoon; and I would like to share my thoughts with all of you.
If you are an architecture professional, then the decision is easy to make. Just buy it! Owning this book to you is similar to getting a good dictionary or thesaurus to a professional writer.
The hard part comes if you are only a layperson or causal connoisseur to modern architecture (like me). To me, the cons to the investiment is twofold: 1)Information overload and redundancy. The information contained in this book is just massive. Honestly speaking, if you have already read quite a few books about modern architecture, nearly 70-80% of these projects (especially residentail buildings) are similar conceptually. What I (and probably you) need now is thought-provoking ideas and eye-catching images, which pop up among the mediocre. As a reference book, this Atlas includes both the ingenious ones and the boring ones. I have been wondering the necessity of reading all these projects. At least, I don't plan to get a PhD in architecture. 2)Size of the book. This point is peripheral comparing with the above, but still important (at least to me). The book is so large and so heavy that I cannot lie in my sofa or bed, holding and reading it comfortably. I have to get a desk and sit tight and straight. Architecture is my hobby and I really don't want to infuse a research-like mood when reading the book.
Fortunately, I picked up another book "Architecture Now 3" (AN) published by Taschen this afternoon. To me, AN is a concise version of the Phaidon Atlas, and could serve as a good substitute. The projects in AN and Phaidon Atlas overlap a lot. The good thing is that, in general, AN selected those interesting ones from the Atlas, removing all other redundant buildings. So you will still get the cutting-edge trends in today's architecture world with fewer projects and a little bit more detailed text. Plus, AN is organized by architects, so you got the chance to know those people (really useful information for amateurs). Further, AN is much smaller and I can read it anywhere I like (on the patio, in the bed, in a park...).
In summary, whether you need to buy this book depends on your needs. Spend money wisely. Hope this helps.