I got the Brady Games guide for GTA San Andreas, and found it invaluable for the side missions where you need detailed maps to locate items or cars. The mission walkthroughs were decent as well, and saved me a few annoying hours of replaying missions that I just couldn't crack on my own!
I'm buying this for those reasons alone, as I'm expecting GTA IV to have even more extra stuff to do aside from the main game missions.
Why would you buy a game guide? Is the whole enjoyment of these games not to complete missions or find hidden items by yourself? If I was you dont waste your money on something that could be found using google!
as the other reply's have said.theyre guides are top quality and best used for when you are seriously stuck,or once youve completed the game going back and finding those last few secrets you missed etc