I used to buy them before I had a laptop and Serebii, PC, and Bulbapedia weren't instantly accessible. But nowadays, I don't. I only got them after I had beaten the game usually, so that I could use the maps to reach places I didn't know about, and check out the Pokedex in the back of the book to get a good idea of future teams I wanted to have and I could plan accordingly. Another reason I don't like them anymore though is because the Pokedex parts of guide books were nuked after RSE. In my DP's guide, the Dex doesn't even have level up movesets listed, and as you know, those are very useful. I just didn't see the point in the guide if it didn't have that information, since it was my favorite part about the guides. Now you have to get a separate book for a detailed Pokedex. I'll stick with online guides and dexes, thanks.
The few I do have though are a Ruby/Sapphire, FireRed/LeafGreen, and Diamond/Pearl. I have some spin off guides too, but eh.