My personal experience with the Millennium File is that it is riddled with errors. I have gotten to the point where I completely ignore the Millennium File records at ancestry.com when I am working on a profile. I have found that if there is an entry in the Millennium File, then there will be other more direct records available to back it up, so I just look for the direct references. If there is an error in the direct reference, at least I will know where it originated.
One example of a Millennium File error had my great grandfather born in Aurora, Illinois. I knew he was from Southern Illinois. I eventually paid to get his death certificate from the State archives and found that someone had misread the handwriting for Anna, Illinois as Aurora, Illinois, and they also misread his wife's maiden name of Morris as Thomas.
The Millennium File just adds an extra layer of human error on the data, and so to save research time, I just avoid it.