fwiw, I rented one to find out if it was something I'd want to own. The D4 sensor is stellar. But the ergonomics of the Df didn't work for me. It's not shaped to where it's comfortable to carry for a long time. There's no grip to it and with any large lenses (like current AF-S G lenses), it's not that well balanced at all. Even with smaller Nikkor AIS lenses, it just didn't feel comfortable after time. Personally, I think this is where they failed with the camera. And also they could have topped it out with the best AF, dual slots (and in a proper place), high speed processor, full metal weather sealed body (like the D4), etc.. In other words, go all out with it. It's already a fairly expensive specialty camera so why not make it a truly premium camera.
I own and use the F3 a lot and it (without the MD-4) is much more comfortable to use. And with the MD-4 it's almost perfect for my hands. I guess I probably have now become just so used to the Nikon digital body paradigm (which started with the F4) and honestly prefer a really good grip, some heft (more stability) and the front and rear dials which I have to admit make for much quicker and efficient use. The D3/D3s/D3x/D4/D4s body style, while larger and heavier, are ergonomic dreams for me especially when using all day long.
After trying out the Df I think I'll just stick to the true 'old school' style with only film Nikons (e.g., F2 and F3) and go with the current Nikon DSLR style for digital. And the Df isn't as small as the F3, it's much fatter, taller, and 'bulkier' feeling. And it isn't that much lighter or smaller than the D600/610/750 bodies (the new 750 is pretty compact overall.)
I think the sensor is the best thing about the Df. If you're really interested in the Df I highly recommend renting one or trying to borrow one to use for a few days before you decide.