Used rough guide to do my trip round SA, found it quite comprehensive and useful. LP have the rep and I would have bought one had they been in stock in the shop that I went to. The writing styles are completely different - LP's much more informal and 'real' in comparison. Guide books are cool to give you the general idea of where to go etc but I always seemed to fine tune those ideas with the info given to me from fellow travellers that I met.
From someone who has never bought a guide book - someone lent me their LP book on Cambodia. I found quite a lot of information really helpful - the little things like dangers and annoyances, what you should know etc. Even saying something like, when visiting the temple, even if you get asked for the 100th time if you want to buy a bottle of water, don't get annoyed or you'll lose respect. Just be polite and say "no". I didn't take the book with me though. The place I stayed at was not listed in their guide book. Then again my friend's copy was an old one - about 5 years.
I've looked into buying a guide book (2) for my trip to Thailand and Laos but have decided I will not buy the one on Thailand - it weighs a ton. They do have "destination" ones though for specific areas, e.g. Chiang Mai, Bangkok, but that means two lots of money. Also the new Laos book comes out in August this year - the 'old' was was printed in 2007. LP do have their pocket book range but by the time you collect the ones you need for the places you're going to, you're still likely to lug around a fair bit of weight.
I have heard it said that many accommodation places suffer the "LP Syndrome" - once they get recommended by LP, all the backpackers who carry their "bible" around head straight for those places. Everybody goes to them.
The other thing apart from the weighty tomes is the price. Forty dollars for a book with information, much of it you may never use or need unless you were scouring every inch of the country. I don't fancy paying nigh on $80 for two books although I may - repeat, may, buy the book about Laos simply because there isn't that much information out there on the net. Seems Laos is "not on the popular tourist destination list." Which actually suits me because from what I've read, Laos is what Asia used to be like before tourism, dollars and greedy business magnates took over.
I find there are helpful tourist brochures and flyers at the airport(s) and/or tourist info centres and I always collect a bundle (I get one of everything), then go through them and work out which ones are the most useful. I did buy a brochure from the GH at Melaka and regretted it due to the size - unfolded out to a very large double newpaoer size. But it does make for a nice souvenir.
You will probably find die hards of Lonely Planet or Rough Guide, and whether you go for LP or RG, I think their days as the be all and end all of travel information are somewhat numbered.