I also have it. It helped me pass all of the licenses.
Plus, knowing the prize cars does help. For instance, I didnt buy and soup up an SLK for the Silver Arrow Races because I knew I would win one. So I bought the CLK for that race, and then I ended up with 2 different cars.
There have also been countless times like that where I make sure I get the best car out of the choices, to help me get every car possible.
The info on the cars isn't worth buying the guide for, but its nice to have. There is also a run down of all the tracks and what to watch out for.
But really, unless you feel you need it. Its just filled with crap, similar to the way a teeny magazine is filled with nothing but eye candy and a small amount of trivial articles.