This book is an excellent overview of the Roman army, particularly the Principate professional army of around 0 AD - 200 AD. The book does cover the earlier Republican army, as well as the later armies (3rd and 4th century AD), but in less detail (about a chapter each, compared to the 8 on the Principate army). The Principate army is what most people think of when they think about Romans, so this probably isn't a bad idea. It's also the period from which we have a lot of information from, which helps make this book both broad and indepth.
It is easily accessible to the average reader, the writing is interesting, and the topics are highly varied. From the soldiers kit and weapons, to battle tactics, life outside of battle, recruitment, leadership, etc., there's a lot of variety in the topics in the book. There are numerous illustrations of good, if not great quality, as well as several photographs of modern archaeological sites or modern reenactors. The latter are particularly nice, as they give a very clear view of what ancient Roman soldiers and officers would have looked like. Pictures of Roman artifacts depicting soldiers, and about 6 two-page color spreads on particular battles, round out the book.
It's hard to criticize this book, other than to say that the coverage is more broad than it is deep. That's a limitation of the size of the book, as each page is quite densely packed with information. This isn't a graduate thesis in the Roman army, but it does make for an easy, entertaining, yet informative overview of the Roman army. Given that it was one of, if not THE, most successful armies in history, this makes this book an excellent overview for anyone interested in Ancient Rome, the military in general, or history in general.