Rough Guide Thailand is one of the best in the range, and it strikes just the right balance, although the book still weighs in on the heavier end in a backpack, its pretty much all useful. especially for the traveller who is exploring thailand, which is after all a very big country.
The books coverage strikes a good balance, and the design is very user friendly, the features of the (not so) new design which are particuliarly useful are the end of chapter transport connections overview with approx times, and the 40 things not to miss photos at the front is a great selection, and can serve as an inspiring hit list.
The book is very well suited to the independant traveller, and the accomodation on the whole was more accurately on target than in many other guides, to agree with 50% or more of the reccomendations is a very good sign, although thailand moves quickly, not least in accomodation, so it is very difficult to keep coverage up to date.
On the whole the food selections were ok, generally this is the weakest point in the rough guide series, Bangkok is least well served in this respect, but when pushed and feeling lazy outside the capital it will deliver somewhere to eat pretty quickly. (not that finding somewhere to eat is a problem in thaialnd).
there is a small but quite useful language section which could be expanded to double its length and be even more useful, and the history section isnt too overwhelming, but i guess more thai phrase books are sold separately than history books and hence the obvious imbalance here.
The Rough guide to Thailand is nicely pitched, accurate and informative.