I use a Df. It is very easy to use pre AI / AI / AIS lenses on the Df, manual focus is very easy to do under well lit conditions. It is very easy to see the sharp details of the subject on the focusing screen when the image is in focus. Under dim conditions it is difficult to see the subject details if it is in focus or not. Under dim conditions I will usually use the electronic range finder.
If you do intend to use the Df for manual focus with the focusing screen, it is best you check the Df focusing screen focus accuracy vs the image captured by the sensor. Some Df focusing screen image plane is a liitle off compared to the sensor image plane, the focusing screen will need shimming, if not the image on the focusing will be in sharp in focus but the image captured by the sensor is usually back focused / out of focus. This problem will be very obvious with f/1.2 or f/1.4 lenses when they are used at f/1.2 or f/1.4.
The focusing screen on the Df is removable (I have removed mine) & can be replaced with a split prism focusing screen is so desired but Nikon doesn't make any replacement focusing for the Df. "focusingscreeen.com" makes replacement focusing screen for the Df, they have range of screens to choose from, from plain matte to split prism type made from Canon & Nikon film SLR focusing screens. Visit their site for further details if you are interested to use the Df with split prism focusing screen.