Little update here.
Wedding season started yesterday and I used the Df for pretty much the entire day. Everything from prep, to ceremony, right through to a dark first dance.
I absolutely LOVE this camera. My D3s's are faster in outright performance as you'd expect, but the significant weight saving using the Df is welcome. AF is perfectly fine now I've worked out how it performs. The handling is great for me and the dials are really lovely to use. But that sensor is the main draw. We know now that it's the same sensor as in the D4s and the colour fidelity at 3200 and above compared to the D3s is very impressive. The files are just gorgeous and there's a rich tonality that the D3s can't match (that's 4 years tech I guess).
I'm a prime shooter which suits the Df. I wouldn't use the f2.8 zooms on it as it would be very unbalanced IMO.
I'll be adding a second one next week and a wedding kit of 2 Df's, 24/35/50/85 f1.4's will save me around 5lbs compared to my kit last season.
Overall, a lot of rubbish has been written about the Df, mostly by people who haven't used one, and much of the criticisms aimed at it magically vanished when people talk about those same things on the Fuji X-T1 (another excellent camera). It's not for everyone I'm sure (the dials will be a dealbreaker for some), but it's a superb camera for discreet documentary work and pretty much the best low-light performance available anywhere.