Never bought one and never will!
When I buy a game I want to figure out and find everything on my own, and not be cheated out of gameplay and exploring by some guide taking me by the hand to show me what I do not have the skill to figure out on my own.
Unfortunately some players buy guides with every game... follow the guide, and finish the game in a weekend, just to claim "I already finished the game!", and become legends in their own minds.
Sorry, but as a hard core gamer since the 1980's I would feel cheated if I had to use a guide to finish or explore a game without using my own noggin!
Don't have the patiance or skill?... then spend the extra 20.00 for a cheat/guide, which I would use to buy another game.
Hold on a second i'm also a die hard gamer, since the 70's in fact, so i know what your saying, however with games as long as 60 hours plus and as vast as they make RPG's now a days, those are the games i like guides for, because with games like Oblivion, Fallout 3 and Divinity 2, you have no way of knowing if you've found everything and done everything there is to do.
So my point is it's just RPG's i just buy guides for, because of the 100's of quests and huge, vast worlds to explore and thousands of items and weapons to acquire, thats the stuff you need a guide for.
I once devoted over 90 hours to Oblivion and still never found every location or every weapon or fulfilled every quest until i got the guide....it was the same with Fallout 3.
So point being, certain game guides are a must for certain games.