it gives the player no real sense of satisfaction, compared to playing the game through, themselves. just about anyone can solve a puzzle game with a walkthrough. if you can't, well, that better be one ****ing hard puzzle game.
i don't, without an exception. for instance, i cheated a few times on the portal 2 arg in order to get golden potatoes that would otherwise have taken me days to achieve. 30 potatoes, still took me 2/3 days. i used a walkthrough to get them all, since, y'know, they weren't defined clearly, for anyone to achieve. (walkthrough or balls-crazy time spent figuring out puzzles and hieroglyphs were the choices, basically.)
when i was a kid, my dad used to buy me game guides for most big-title games (yoshi's island, mario rpg, banjo kazooie, etc) so i would look at them for help when i got lost, etc. i think this is about the only time that "cheating" should totally be encouraged, as it also helps promote reading, in a weird, offbeat way. i mean, you can always fumble through using pictures, but you're not going to know which door and switch combo to activate unless you can read properly, right?
i dunno. i'm against it, pretty much every time it's unnecessary.
"fun" cheats and stuff don't count, since those are plainly described: "fun".