I am an avid Lonely Planet fan and a pack rat. I have every single Lonely Planet guide on one book case.
Lonely Planet only updates their books every three years, sometimes two. So some destinations I have three separate editions.
For what I need when I travel Lonely Planet is the best guide. They have great history sections and great tips like how much you should be able to bargain for a cab from the airport to the city.
I don't use guides in the States and Europe as I am always staying with friends and going to familiar places.
One day soon I will have to throw away some of my older guides that are completely useless.