I must admit I have not read all of the posts on this thread, so for some I may be going over old territory.
I started shooting seriously about 40 years ago, and when I began to do so I got a couple of Nikon F3's and Canon A-1's: both arguably leading edge at that time. I appreciated each camera for its specific benefits and frankly at the level they sat at it was more of a case of horses for courses rather than one being demonstrably superior to the other.
When I went digital, I chose a single brand, and I did so on the strength of the glass, rather than the body. Frankly, for a period after the release of the Canon D60 until fairly recently, Nikon has, with some justification, claimed the better sensor. At the time I chose Canon, the Canon EF 70-200 L 2.8 IS USM MkII was a major draw card and I wanted the access to the vast array of lenses offered by Canon. Since then my collection has expanded to about $90k worth of gear, of which Canon represents about 95% of that - so I have a lot of skin in the game and I am certainly a loyal Canon client. I made an exception for the Nikon Df based on the body, and more specifically the interface. However, I also wanted digital performance and found it in the awesome sensor (adapted from the D4) that smoked contemporary Canons in low light performance.
The following submission was presented to a YouTube review that was written by someone who had never even used the camera, yet felt qualified to express an "expert opinion".
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The Df is an amazing camera and was hammered by people who have no or minimal contact with it. This is a camera that CAN be used as as digital DSLR, or it can be the digital version of the film cameras I used to use back in the days when I first started as a photographer. You can choose which personality you choose or apply a combination of those two interfaces.
Its intent is to re-create, as much as possible, the experience that photographers had when using a film camera. But it recognizes that digital photography has other elements that film did not have and it has tried to deal with those without losing the analogue interface. The experience of using film required a discipline of approach that one does not have to have today and what some regard as shortcomings, I see as a recreation of those conditions, and I'm fine with it. The clues to the fusion philosophy are in the whole design ethic:
1. The ability to use Non-AI lenses
2. The use of analogue dials controlling the essentials
3. The fabulous sensor, upgraded with a new processor to improve low-light/high ISO performance. It encourages you to use available light and fast prime lenses.
4. The removal of video to concentrate on stills, (according to Nikon) making the camera more compact.
5. The lack of alternative focusing screens
Let me address some of the criticisms I have seen hurled at this:
FIRST: It's a STILL photographers' camera - that deserves no apology, there are many DSLRs out there that do video just fine. Nikon F-series cameras were still cameras.
SECOND: It doesn't have a built-in flash. Neither did the film cameras, but it has a perfectly serviceable flash hot shoe with all the capabilities of any Nikon camera built-in.
THIRD: The unit does not have enough focusing points. It has a lot more that film cameras did and it works fine if you know how to use it.
FOURTH: There is only one card slot. Film cameras could only hold one film at a time. In the days of film when I was shooting around NZ, Australia and Asia for landscape, wildlife and travel production I could carry only a limited amount of film and that had a finite life in the very hot conditions. When I took a photo I would not know if it came out for maybe a month before it was developed. The temptation was to take several bracketing shots, but then there was the limited film capacity to consider. It generated a discipline of being sparing and very careful with my settings and composition. I still do that today with digital and shoot a lot less than my contemporaries who only knew the digital environment.
FIFTH: The controls have lock on them - yep and so did most of the film cameras, it's about learning to get used to them, once you do it's automatic.
Sixth: Lack of focusing screens - there are some available via 3rd party suppliers, but considering I use auto-focus lenses I use spot focus and let the system do its job.
To me, and those who value this camera, it is all about taking time to enjoy the process of taking a photo, as well as the final outcome. In a similar situation my daughter's boyfriend asked about my record turntable and asked why I would still have one of those when an MP3 player was much more efficient. My response was that playing a record becomes an occasion in its own right and that was a big part of the enjoyment for me - in exactly the same way as taking a photo with the Df does.
I have now retired from my photographic career, I take photos for free and for me. I still have over $90k of Canon gear, which I have used since I went digital and I shall continue to use it. I chose Canon digital for its glass, but I always respected Nikon - I used them both when I shot film. I chose to switch brands for this body alone because of what it is specifically and I am happy that I have done so.
There are a lot of photographers out there who crave the latest technology on the belief that a better camera will make them a better photographer, or that the gear is somehow holding back their innate abilities. In 40 years of photography I have never felt constrained by the gear (I have used Nikon, Canon, Olympus and several other medium format brands). I have felt constrained by my skill in using what I have. For those who want the latest tech this is not for you, move on and be happy. For those of us who value the process this is a fine camera and worthy of respect.
The period around the release of the Df was marked by a lot of vitriol aimed at the camera, much of which was based on bias, tribal brand loyalty and ignorance. Since then the Df has established its place as a niche camera with a loyal following from those who appreciate its particular strengths.
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Regarding the debate on the relative physical attributes of the Df vs. an equivalent Canon.
NOTE: I am comparing the Df to the 5DMkIII as they both have professional-quality FF grade sensors in them, the EOS 6D did not. In the case of the Df, it has the same sensor from the highly respected Nikon D4, which was, at the time, Nikon's flagship camera. The Canon 5DIII was Canon's flagship camera.
Links to comparison of the sensors of the Df, 5DIII and D4 by DxOmark: https://www.dxomark.com …us-Nikon-D4___925_795_767
show the sensor performance for the Df and D4 are consistently the same, except the Df had been tweaked to significantly improve low light performance, while the 5DIII lagged behind across the board.
The Canon 6D was an equivalent to the Nikon D600.
According to DPReview the following apply to the Df and the EOS 5DMkIII
Weight (inc. batteries) Nikon Df 760 g (1.68 lb / 26.81 oz), Canon 5DIII 950 g (2.09 lb / 33.51 oz) so the Canon is 190gm (25%) heavier
Dimensions Nikon Df 144 x 110 x 67 mm (5.67 x 4.33 x 2.64″) Canon 5DIII 152 x 116 x 76 mm (5.98 x 4.57 x 2.99″)
While at first glance the 5DIII and the Df seem to be a similar size. However, while dimensions are absolute, they don't tell you everything, for example how the camera body is shaped within those maximum parameters, so here is where some photos add to that perception.
From these images it is evident that the bulk of the Canon is greater across a much wider area.
I had a 550D before the 6d, and after using the 6D, I actually like the size of the body more, being that bit larger. At the risk of sounding like a fanboi, I find the 6D to be the Goldilocks camera for me 
