I'm a Lonely Planet junkie. As others say, use your eyes and brain too, but they have a good range of price points; mix up easy/must see sights with some more adventurous expeditions where you don't meet hordes of fellow tourists; they point you in the direction of good food; they offer some basic phrases in foreign languages that can break the ice with local people; they have maps; they have top tips on avoiding scams; they have typical prices; and the list goes on. Whilst I would avoid a restaurant that claims to be listed in Lonely Planet, most of their listed restaurants don't advertise the fact. They just get on and do what they do best.
On the other hand, I remember being in a couple of places without a guidebook, and you end up just wandering and wandering, aimless and bored. Or you just land up in a bar, drinking to kill time. Tourist information offices just send you to the very, very touristy places (as do most local people). They won't show you the hidden gems, or explain what you are seeing. Restaurants that you chance upon might be good, but IME they might equally well be dreadful.
I had my eyes opened when a friend came to stay here in Edinburgh and brought a Lonely Planet. Reading about my own city, I discovered all sorts of places I never knew existed.
And I've just got back from a week's holiday in Algeria.
In Algiers, I stayed in a hotel I found on the Internet, but used LP to help with food and an excursion on local buses to Tipaza. Without LP, I couldn't have done this - the locals all told me to get a taxi. Not that I'd have known about Tipaza anyway without LP.
In Ghardaia, I stayed in a great place listed by LP and wouldn't have found it (or anywhere else) as Ghardaia isn't really on the web.
