I agree with Andy Hollis. I have driven the Nurburgring during 3 separate trips with Ross Bentley and Tom Roberts SpeedSecrets group, renting the Ron Simons RSRNurburg cars (including the group Andy was with this year). Now having logged over 100 laps.
Sim: I started using iRacing 6 months before my first trip, continued to use it prior to my second and third trips as well. It was invaluable and necessary. The sections beginning with Wipperman, through Eschbach, Brunnchen, Eiskurve and ending with Pflanzgarten, a driver can become "lost". So, a sim is a must.
TF: I agree, avoid the TF sessions, unless during Sunday mornings (when most have been up too late Saturday night and the track is empty)
Rain: Agree, do not be afraid, but be aware and respectful. It can be sunny at Tiergarten and pouring down rain at Bergwerk. So, be prepared - it is a 14 mile long road course with 1,000 foot elevation changes. Our second day this year (Sep 1) was one of constant rain. I drive my home course at VIR in the rain quite a lot, so rain is not an issue for me and enjoy it quite a bit. But, having been my third trip (and driven in rain during one of the prior trips), I elected not to drive that day, after deciding I did not want to pay for a rental car I could not bring home.
For comparison, we drove SPA-Francorchamps during the same trip. I found SPA, having been modernized for F1 purposes, to be much easier to learn after a couple laps and not as challenging as the 'Ring. SPA has ample run offs, with gravel pits, so no real risk of damaging the car in comparison to the 'Ring. Remember, it takes over 3 laps at SPA to cover the same distance as one lap at the 'Ring.
Fellow drivers: Experiencing the Nurburgring with 20 fellow enthusiasts in the SpeedSecrets group creates some long-term memories and friendships. Hard to find an equivalent experience.