OK first of all I have to say that I just used the Lonely Planet, so I don't have any comparison. But in comparison to Lonely Planets of other countries I was not very happy with this one.
First of all, many maps are inaccurate, what is really problematic if you are e.g. looking for a certain hostel and you do not find it because the map is wrong.
It is not new that even Thais do not know when buses leave, but why does LP print many departure times - most of them are wrong, of course, because the timetable seems to change daily. You do not realize it at once an spend a lot of time at many bus terminals. Many chapters are too long with stuff you do not need, and many really interesting places are mentioned in just a few words. One example: the National Parks, Thailand is a land with many NP, and important hints are not given in LP, e.g. Khao Yai NP is very large, you have to hitch, or rent a motorbike in Pak Chong, it is just a sentence that can help so much. In Khao Sok NP you do not need a morotbike at all (it is funny to go a hundred Km to get there and realize it) - OK perhaps not everybody likes NP that much as I do, but on the other hand, Bangkok has really many pages in the LP. Some large towns are not mentioned in the LP, e.g. Nakhon Sawan, Suphanburi - why not. Ok they are not on a tourist route, they are of the beaten track, but they are interesting and worth a visit, because they are of the beaten track, and I haven't seen a tourist there once. Isn't LP supposed to be a guide for people that are searching for adventures :-)(it says so in the foreword) - thank you LP, because if you mention it I would probably have seen one or even more tourists. But I think there are more towns like that, (I recommend the North-East if you cannot stand Westeners for a time).
One the one hand, LP was OK but not worth the money and has to improve. The normal tourist tracks get too much space in it, and they are not really worth it. On the other hand, I do not know any other guide that could be better than LP, or I cannot remember one. Therefore I can just say, if you want to go to Thailand and use the main routes, LP is just OK', is the only choice. Have fun in Thailand and do not forget to smile. Kop koon Krup