I usually don't buy the guides. If I get really stuck, I look it up online.
It also just seems to spoil the surprises in the games. When I go into a new region, I like wandering in the grass to see what will pop out at me. This is why I was like "ZOMG!!!!!" when I got jumped by a Chansey in the first half of Diamond. When you buy the guides, they tell you what Pokemon are in the regions and that makes it less fun for me.
My obsessive need to catch every Pokemon I don't have in the Pokedex, even if I never use them, made me waste all my Great Balls and some Pokeballs on that Chansey. It took over ten tries to finally catch it, and I had to run away the first time to get my Hypnosis Hoothoot out of the PC. Thus, another fifteen minutes of searching for that elusive Chansey.
When I finally did catch it I felt really successful. I checked it out in the PC and it had some pretty good moves. Then I realized I had a level 16 Pokemon with over 100 hit points and I was like LOLWUT! O_O =D