Well that depends on the user.
I spent 8 days in Rome and carried this book with me to many places. It was especially handy when tours were unavailable or inconvenient. Testa blends interesting tidbits into a thorough, yet concise, summary of each subject. If you're not rushed for time and have a relaxing 15 minutes to sit outside of St. Peters, or the Colosseum, or one of the other 35 places/topics included in this book (or even later in the hotel), then I would recommend it. This is indeed where the book proves its value.
If, however, you are rushed for time or deeply interested in one particular topic, other more general (Eyewitness, Rome's Top 10) or more specific books (for all sites covered there are probably many books to chose from) may better suit you.
Other travel guides are best used for planning purposes. Leave them in the hotel. They're too heavy to carry. Though not billed as a travel guide, this book works really well as just that. This is true *only* if you are the type of person who will take the time to read it. Otherwise, save your money for the gelato.
Finally, and this is my only criticism of the book's content... or the lack thereof, I was a little disappointed at the breadth of coverage. It seems (again considering the price) more subjects could have been treated. For instance Santa Maria del Popolo was not among the subjects included.
But this opinion may be a product of how I used the book. It is apparently intended to be a survey, not a comprehensive guide. My review however, is written with the traveler in mind. (If you're just interested in a general survey of Roman art and architecture, I've only read this one. It seems adequate enough.)
Not until I read Testa's, Rome is Love Spelled Backward, and A Traveler in Rome, by H. V. Morton, did I come to understand and agree with the quote, "The ancient ruins are all around you, . . . . This is the center, and all the rest of the world is the periphery." Judith Testa's book is the only book I carry with me while wandering around Rome, year after year. A pre-Roman holiday must read!