Technically, I believe Forza 5 is the first game to feature a laser scanned Nurburgring. IMO it shows. Some of the kerbs seem oddly wide, as they have in previous Forza's, but for the most part it seems spot on. The trees are are really shitty, two dimensional things, but there's so damn many of them and you're moving so fast that you'll never notice. The whole thing is very impressive.
So far I've only done one lap of it, in the Brabham BT24, and it was absolutely terrifying.