The most grip is currently at 0.0, any camber added will reduce potential grip but can be used to make steering less aggressive or intentionally cause slip (easier rotation) in the case of rear camber on FF cars. It has no positive effect but can be used to flatter your driving style if you prefer a less aggressive turn in or less grippy rear on certain cars.
The current verdict is that camber is either broken or we haven't figured out how to apply it yet, it certainly doesn't have the same effect it did in GT5.
If a car comes with camber as stock (most racing cars), set it back to 0.0 to gain grip.