It all depends on the guide, really.

Some are horrendous like the Final Fantasy IX guide (go to Playonline.com for more details) and some are excellent like the Lunar and Lunar 2 guides.

If the guide is great, then by all means I would recommend purchasing it for a second playthrough of the game. The Lunar guides are by far the best strategy guides I've ever seen. They have everything from maps to funny captions written under in game screens to comics. And of course they have the number 1 most important thing ANY game guide should have: Boss HP.

I have the Final Fantasy X-2 guide and it's pretty bad. The Final Fantasy Official guides usually aren't that good. The X-2 guide completely skips the Bevelle Dungeon.

I usually like to go with unofficial guides. There's this one unofficial Final Fantasy VII guide by some guy named Casey Lowe that is extremely well done. It even came with a poster.