I usually go with Lonely Planet rather than the Rough Guide since I tend to feel it has the edge on accuracy. But the Thailand guide is a little disappointing. The Bangkok and Chiang Mai chapters are fine and full of useful and correct information. But chapters covering the smaller provincial towns - Phrae, Nan, Lampang - are less reliable. Particularly, the maps just don't look like the towns, sometimes I've ended up walking for the best part of an hour to cover a distance that looked like a kilometre according to the scale on the map. Also, lots of important streets are merely drawn, not named on the maps. The combination of these two failings means you can get lost very easily.
I'm not going to complain, like a previous reviewer, that the book assumes you can speak and read Thai. The authors always warn you if a particular restaurant has no English menu or sign, for instance, so if you can't read the language, you know to choose another restaurant. I do think, though, that the authors should take into account that prices tend to be much higher for lots of commodities - taxis and tuk-tuks, above all - if you happen to be a non-Thai-speaking foreigner. I've set out a couple of times on a journey which Lonely Planet has assured me will cost 30 baht by public sawngthaew (pick-up truck), only to arrive at the bus stop as the only customer and find drivers insisting that I charter the vehicle for 500 baht. You would need good spoken Thai and good haggling skills to avoid these sort of situations. Of course if you travel to a country where you don't know the language things are bound to be difficult at times, but I think Lonely Planet should take those difficulties into account.
Of course the chapters on Southern Thailand will now have to be updated in the light of the recent tsunami tragedy, not to mention continuing unrest in the Muslim-majority provinces in the far south. But no guide book can keep pace with events such as those.