Okay, firstly, I'm a little late on the GTA:VC thing. Only just bought the PC version in November 2004. At any rate, after being faced with the myriad of possibilities within the game, and the challenges that the many missions and rampages introduced, I opted to purchase the game guide to help me along to the coveted 100% completion.
I typically buy a game guide for any semi-complex game I play on my PC. But this one by far has to be the worst. As far as the missions, rampages, etc are concerned, the guide offered little information over what could be found online or within the game itself.
There is however, a complete guide to all the vehicles, which may be handy to some, but not really for me. Missions are somewhat detailed, with accompanying maps, but generally contain wordy overviews. A series of maps round out this flimsy tome, all of which can be found in various incarnations online.
One glaring oversight is a map/chart for all controls. The bulk of the info you find online is related to the PS2 version, and the manual that ships with the game itself lacks detail. In particular, controls for helicopters never seem to be mentioned in the guide. I'm still trying to find out how to increase my health by means of the local prostitutes.
Overall, it's nice to have a book (albeit a slim one, only 192 pages) to refer to while playing the game, but it lacks any real advantage over trial-and-error, or by looking info up online. I'm just glad I had a $5 Amazon gift certificate, otherwise I would have felt a lot more cheated.