My upgrade came was delivered today. My initial impression is the improvement over the 645AF is significant.
The AF is much faster and so the lag is much shorter. The tactile feedback on all the buttons is certainly very welcomed.
What I like most of all is the more accurate focus that I'm seeing. Wide open shots of a target that I have shows sharper images. As the 645DF has the option to set AF as "accurate" or "faster", I opted for accurate. however, I could not discern the supposedly slower AF speed when using this option. I'm leaving mine on accurate. I also found the audio beep feedback to be louder so when AF is achieved, I hear it distinctly, even when using MF.
Overall, I would not hesitate to fork out the $1500 to upgrade from 645AF to this new body.