I am a big LP fan (I definitely prefer them to Rough Guides) and this book is up to LP standards. AS with other LP multi-country guides (e.g. Guide to the Middle East) this book is great for starting off your country by country research, giving you an idea of the key places to visit, guide to costs, travel times/distances etc. I won't, however, be using this guide to help me travel whilst in the countries for the main reason that the "Getting to and from" sections that LP readers will be familiar with are either lacking in detail or just lacking all together! The information on more remote sites is also limited (major sites like Angkor Wat/major cities have plenty of detail).
I would definitely recommend this guide as a start to your travel research OR as a travel guide if you only intend to visit the major sites in each country and have your travel organised for you but I wouldn't recommend relying on this guide for more extensive travel once in V,C, L and NT. Invest in this book but also buy the single LP country guides, they will give you the detail you need to organise your travels once in the countries.