while traveling through cambodia and vietnam I actually had a couple different guidebooks. overall I think Let's Go's guide is the better resource, and especialyl for a traveler who's covering a lot of ground in southeast asia. if you use it in conjunction with another guide, I think, you will really get a good feeling for what to do. i would not advise anyone to take just one guide, no matter what, or to use Lonely Planet alone, which was really god awful.
the information in my Lonely Planet was laughable, as many of the maps were lacking. and in general i think LP's maps and info aren't as clear. While Let's Go could stand to give more options for different establishments and price ranges, the quality of its places are generally higher--with a few exceptions-- whereas LP just lists every single place regardless of whether its a hell hole or not (you will learn this very quickly)
though I don't usually have an allegiance to a certain brand, I'm pretty sure I'll take let's go with me when I go to India this summer, too. it seems that for the developing world, its a rock solid resource.