I find very little of real use in most guidebooks. I'm quite big on planning, so I tend to read everything I can before I go, but these days I get as much if not more off the net, or from tourist information offices, as I do from the guide books. I photocopy what I need rather than take a whole book away with me, but usually it's only a few pages.
City guides I find a lot more useful than the outdoorsy ones, especially the Blue Guides (not as racy as it sounds) series.
The only walking guide I've ever got any real use from is the Kummerley and Frey 'Hiking in the Jungfrau Region'. Simple, factual descriptions, topographic diagrams and a broad spread of routes all packed in a tiny phrasebook-sized package make it 10x more useful than any other English language competitor. Unfortunately this is the only area they do an English guide for. I agree that the Cicerone-type guides, including the Lonely Planet 'Walking' series, are pretty awful. For Europe generally, I find German-language guides are generally far better than their English equivalents, but obviously the language barrier prevents most of us getting the full use from them.
When it comes to further afield, the Footprint South America Handbook is the only one I'd bother repurchasing. Apart from the scarcity of genuinely factual information, I think the big failing with most of these guides is their insistence on structuring their content by region, which necessitates far too much dull repetition. Sparsely populated areas, like most of Nepal, I think would lend themselves far better to a thematic treatment.
I don't think guidebooks are the necessity they set themselves out to be, anyway. On my first trip outside Europe, to Thailand at the age of 18, we had no books, maps or remotely suitable gear. Not a conscious choice, this was due entirely to ignorance and stupidity. Nevertheless, we not only survived but enjoyed ourselves for two months (including a month in the jungle), and unbound by guidebook 'don'ts' had many experiences we perhaps wouldn't have been open to otherwise. Of course, this sort of approach isn't always viable, but neither should it be ruled out, especially if you've got a relaxed schedule and an open-ended timetable.