I find it hard to rate GT games because I never really consider them to be in the same category as your average game. It seems, though, that in their efforts to make GT a better "game" (what with the levelling system etc.), PD have detracted from the brilliant physics model and beautiful attention to detail.
GT to me has always been about the cars, and the latest iteration feels a bit diluted in this regard. The whole standard/premium issue doesn't really need any introduction and other elements such as the tuning and the AI don't seem to have improved at all. If anything, the AI seemed better in GT5P.
Anyway if I were rate it as a "game" in the traditional sense, I would give it about a 7. But if I were to isolate the best aspects of the game and, say, just lap the Nordschleife all day in premium cars only, then I still have to give it a perfect 10. It just doesn't get any better IMO