I tend to buy guides more for the official artwork than the actual guide part. I usually use them for second playthroughs only unless its something like MegaTen where having a fusion chart can save time and controllers... as Jiro said. I tend to want the first experience of the game to feel as natural as possible, and I feel having a guide telling you how to do everything and where to go to get overpowered items tend to cheapen the gaming experience. In fact, its because RPGs and some games tend to put in game-breaking items into their games that I tend to avoid using guides.

I mostly use them for the second playthrough cause I play mostly for game mechanics now that I know the plot and don't feel obligated to rush to each new story segment. That's when I do that perfect file, though some games tend to be elusive...