Fallingwater hasn't been a private residence in 50 years. The Kaufman family turned it over to the Western Pennsylvania Conservancy -- and I'm sure got a hefty tax deduction. The Kaufman's actually spent very little time in the house. It was built as a summer home and used through the 40s and 50s as such. The house itself is cold and drafty with high moisture. The kitchen is about the size of a closet.
From a maintenance standpoint, it is a money pit. They had to rebuild the upper patio which was in collapsing.
I have visited it a number of times and it is wonderful to look at, but as far as a residence, I much rather have the guest house which is far more comfortable than the main house.
Since you have visited there alot, here is my other gripe (other than the coldness/dampness) I didn't like (I know taste is subjective) the built-in furniture. Almost everything was/is stationary...I remember being on a tour and remarking (quietly to my friend) 'wouldn't you like to change a room around?' only to be stared down like 'you are for real? Stunning setting and great ambience, but you are right...not 'living in' friendly.