My fiance and I recently spent a couple of months touring Thailand with this book in tow. For the most part the book served its purpose. However, there were times when the book was unnecessarily frustrating.
For example, those cities in which no map is provided, the descriptions of the cities are virtually worthless for navigating. In more than a few cases, restaurants or accomodations listed in text are not included with the corresponding map. We often got the impression that first-hand research was not done for some of the activities or accomodations - mostly for southern Thailand. For example, a describedly tedious 1-hr "rocky jungle" walk from Ko Phi Phi's Thon Sai Village to Long Beach actually takes about 10-15 minutes, mostly on a fairly wide dirt road. This leads me to believe the researcher took the word of a longtail boat driver, looking to sell a fare, instead of doing the needed first-hand research. More commonly, it became apparent that rather inconsistent reviews of accomodations were likely done either superficially or lifted from previous versions of this book.
Overall, the book is better than Lonely Planet for budget travelers (which is too broad). LG does lack in background information concerning many of the sites. However, it is a pretty good guide about how to travel around and where to stay. Some of the side trips are nice (Wat Phutok in Northern Thailand is a must if in the neighborhood). Even when there are times when a little more attention to detail, consistency in reviews, and correspondence between maps and text would have been appreciated. This book will generally get you to the right parts of town, in the right part of the country, for about the right price, and with a decent map for many towns. Where it can be frustrating, well, mai pen rai.