I had a ten lap race on Nordschleife in GT5 a while back that stands out as the single greatest simracing event of my life. Me and a friend drove in the early morning going REALLY hard all ten laps. That's almost a GP distance and took longer since we were in Mitsu Evo X cars. We were door to door for a very large portion of that race, and swapped positions so many times. We finished within a second of each other, and both of us went very close to 100% effort throughout.
That was truly satisfying, and a lap of the real thing feels kinda the same in many ways. Concentration is so high, g-forces and adrenaline make for a thoroughly unique experience. No other track I've driven require the same level of concentration and effort to be really fast on, and there is no way to have as intimate knowledge of every piece of the tarmac that you can on other, shorter tracks.