2010 - 8.0/10
2012 - doesn't matter
I don't agree with people judging the game twice, with the second being a rating of what it eventually became after a couple years of updates. Games are and should only be rated on release day. If a movie hits theaters this weekend and it sucks, we can't judge it higher because maybe a couple years down the road a director's cut will be released on Blu-ray with edits making the film somewhat better. What it is upon release is what it is and what it gets judged on. Besides, if it's going to get judged post-updates, then it also needs the rating scale to adjust for comparison versus games released since its release, namely FM4, which improved upon FM3 pretty substantially, and FM3 rated above GT5. So, if GT5 is to gain fractions of a point because of its updates, at the same time it must lose points for falling farther behind the Forza series during that time period.
Why did I go with just an 8? Well, it only has 220+ GT5 car models, versus FM3 which had 400+ cars (not counting subsequent DLC), and note that this ignores FM4 and its 500+ cars (not including DLC) that came along a year later. Some of the tracks are last-gen models, too, which hurts. There were some graphic bugs holding it back a bit there. The menu design was horrid. Basic stuff like painting my car was unnecessarily made a tedious chore. They wasted the Top Gear license. Importantly, the game was missing features while only boasting (versus the other guys) stuff that was poorly implemented (track editor could produce tracks that we couldn't share with all 6+ million players). The game can't have all these strikes against it and still rate closer to Forza... unless we're going to score Forza higher than 10.