Well, I didn't do the "sunset tour," but I did take the in-depth one, and was happy to have done so. It was great having only a small group, all of whom were pretty serious about the house, and a bit more time. The kitchen was interesting, the staff rooms (which are now offices and retain only the original wood cabinetry) not very. I'd been warned about the interiors and seen other Wright homes, so my expectations of the interior were pretty low. I actually liked the great room better than I expected, but for me nothing in the house (either looking inside or outside) was as great as looking at the house from the Viewpoint.
I did have Hertz NeverLost, but heeded the warning on the Fallingwater website and managed to get it to give me the same directions to Fallingwater, though it told me I had arrived about 1/4 mile too soon. It did not have Kentuck Knob in its system, so that was useless. Then it sent me on 10 miles of winding road with a 45 mph speed limit as the "shortest time" back to Pittsburgh, so I am not sure GPS is the answer.