Having a hard copy of a computer book in hand when learning a new technology is a great thing - when the book is great, that is. However, Practical Symfony is not a great book, and given that it can be downloaded from the Symfony site, I do not feel after buying the book that it was work investing in a hard copy.
Symfony is a decent framework for building a Web application, and as lead developer of the Symfony project, Potencier certainly has the expertise to provide correct information about the subject. However, Potencier's expert knowledge often gets in the way of teaching the technology, and important subjects are glossed over and some items (such as how to display the debug toolbar) that may have seemed too trivial for inclusion by the author. For instance, the Doctrine_Query object is given half a page of discussion instead of the full chapter it deserves. One simply cannot get an adequate understanding of Symfony and its integration with Doctrine from this book alone. I find myself using my Google-fu and reading pages of online documentation for both Symfony and Doctrine just to figure out how to do basic things that I could have done easily outside of the framework. A book should effectively minimize this, but this one doesn't.
Practical Symfony is written in a very linear format. It is structured much like a SAMs "Teach Yourself... in 24 Hours" book, and suffers from the same structural defect as the worst books in that series. If you have 24 days to devote an hour or two to each day to a chapter, and you think up your own simple project from scratch and follow the book's process, you will learn the basics of the technology. However, if you are like me, and are trying to take over an existing project written by someone else and extend its functionality, this approach does not work. For working programmers, books that have solid chapters that can be read coherently by themselves are much more practical and deliver far more value, which is not the case with this book.
That being said, what ultimately cripples this book's usefulness is its abysmal index, which appears to have been generated by computer and not subsequently reviewed or tested by an actual human beings. For instance, there are two entries side by side for "Web Debug Toolbar" and "web debug toolbar", which point to different pages. This case insensitivity problem is indicative as well of the numerous capitalization errors in the text of this book.
Finally, this book is a translation from French, and sometimes the resulting English is cumbersome or incorrect. One frustrating example is the use of the word "serialization" to describe the process of Doctrine writing data from a PHP object to a database. While this process is conceptually similar to serialization. I found it highly confusing to see the term used in that context, which to me implies the use of the serialize() function or a comparable routine such as json_encode().
My conclusion is that if you want this book, download the PDF from the Symfony Web site. If you want to support the project, click the "Donate" button. However, save a tree and don't buy a physical copy.