I have been driven the Nordschleife 4 times, 3 times in my 165hp Nissan Primera and once with my uncles supercharged 540i BMW. This was before GT5, but I had already done 100's of laps on GT4 before my first time. I would say given my knowlege of the track from the game, along with watching numerous onboard videos online I was quite confident going into it. This quickly changed with in the first couple of corners as I started realizing the track is really really bumpy and has tons of wavy parts that were making me feel unconfident about "knowing" the track right away, so I ended up taking it slower than I wanted, I had 2 friends with me as well so it ended up being a fun but cautious run.
Then in 2008 I returned after having closely studied serveral onboard laps with real drivers commenting about each corner and the dangers of the bumps, elevation changes and angles of the track, and this really helped me with my confidence and I would say I was driving it a lot more like it should be driven and I was getting the tires to squeel through a couple of turns. I learned a lot in that 1 run. A week later I came back with a friend and gave it another good go, he thought I drove it extremely smooth for only being my 3rd time (he had drivin it about 10 times already in his Golf). Definitly tons of fun, but take into consideration that I also have alot of experience racing on small tracks and autocross events, as well as driving hard around empty roads and learning how the car feels. I moved to Canada like 2 months later, so my Nürburgring career was over... lol. If I move back I'll probably buy a season pass so I can have fun all year.
Last year I went to visit and convinced my uncle to drive down (1.5 hours) with his BMW and we had pre-purchasing 2 lap ticket online. He's no expert when it comes to understanding the balance of a car so he wanted me to go first and he immediatly panicked around the first corner, he didn't know his cars can go that hard around corners and be under contol...lol. I had to slow down for the first few minutes and take it easy so he could get used to the feel and of the constant turns, but I still drove pretty hard towards the end to show what his car can do. I got his respect for sure and when he hopped in for lap 2 he was so paniked about the track, he drove so slow, and was like "I don't want to drive fast".
He knows that I've been on it, but the only reason I am so confident about it is GT and watching onboard vids. Now my cousin who is 12 yr. old is playing GT5 because my uncle went out and bought it and they got the DFGT a couple months ago.
That's my story, and I would recommend to anyone to löearn the track via GT5, because there are too many turns to memorize. They've done a good job on GT5 for sure, but it's probably slightly off (only by mm) unless they laser scan the track like the guys at iRacing do. And unless you have extensive RL racing experience I would not recommend relying on video games to drive through the "Green Hell" too fast.