If you ignore some of the laughable reviews on Metacritic, the average probably goes up to around 8.
The NowGamer (never heard of 'em) review states it's not a worthy upgrade to GT5 (I'd say the much snappier load times alone make this a worthy upgrade), then proceeds to give it a 7/10 score despite the average of their category scores (graphics, sound, gameplay, longevity) equaling 8.125.
VideoGamer's (ditto) review, the worst of the few that I've read, complains about it being on a now-outdated console, nitpicks non-issues with the menu system and ultimately concludes that GT6's new menu isn't streamlined but is in fact "convoluted" like GT5's, and constantly laments the apparent fact that GT6 has 101 Mazdas in its car roster. He also complains about the barren landscapes, presumably preferring them to look like NFS Shift's circus tracks. He ends his review by stating "Played for 11 hours. Drove quite a few Mazdas.". Wow, what a review.