I've just got back from Rome, and this book was my guide the whole time I was there.
It has enough information within it, to ensure you know the important aspects of the many historical sites. It also has great listings on eating out, and where to stay, and lots of snippets of information that will come in handy. This guide stayed in my ruck sack the whole time I was there (I enhanced the maps at the back of the guide by purchasing a Lonely Planet laminated fold away map - great when it starts to rain).
As for Rome itself, what a beautiful place, I stood in the colleseum on an upper level directly opposite where gladiators would have made their first entrance into the arena, at the start of the many games held there, and read the section from the book to my friend, and a few others were listening in and we all started to talk about what the book has said...so yes, it is worth the purchase, and I would strongly recommend it as a guide.