Like most of the Frommer series, this book is very strong on logistics. The details on transportation, hotels, travel bookings, vaccinations etc. are very strong. It also has a chapter on Angkor Wat which makes it very convenient to take as a single pocket guide for a trip that includes both Vietnam and Cambodia. The information on avoiding certain tourist traps and avoiding taxi frauds are invaluable.
The weakness of the book is that it is short on understanding the place and culture you are actually visiting. The author seems to have got most of the perspective from expats instead of local residents in the 2 countries. The comments on restaurants are littered with references to "this is a good expat spot" and it shows in the resulting quality of recommendations. This clearly does not help you get an authentic cultural experience.
Similarly, the discussions on the various places to visit are also slightly superficial. There are various mistakes. For example, the Quang Cong temple in Hoi An worships a Chinese general from 2000 years ago from the Han Dynasty, not from the Qing dynasty as indicated by the book. The descriptions of the various temples in Angkor Wat does not contain a detailed enough description to give you a feel for what to expect. It also have its share of grammatical errors / typos such as "losing site of...." and "Vietnamese gorillas attacking American troops in South Vietnam."
All in all, this is still a great guide for its practicality. To maximize your experience, you would need to supplement it with better books on the culture and the sites you are visiting.