I used to buy Guides because I hated getting stuck (especially by a puzzle or something) and not being able to continue/finish. That was before game forum/sites were all the rage and it wasn't always easy to find a quick answer. Plus, I liked the ones that gave formulas and percentages and other geeky stuff. :D
As long as you don't read the Guide, play, read the Guide, play...nah, doesn't ruin anything.