Launched within weeks of one another, the Nikon Df and the Sony A7/A7r could not be more different cameras. Even though they are conceptually "smaller" full frame cameras, the design that each camera is built off of shows that there is more to a camera than just its sensor. The differences are quite stark when the two cameras are put side-by-side with their respective kit lenses.
The Sony is obviously the modern of the two, but even though the A7 and the A7r are visually similar, even within the same family these two cameras appeal to a bifurcated audience. The kit lens gives away the A7's target audience; variable aperture, sacrificing smaller overall size for convenience. In one sense, it's a spec chaser's camera; a lot of people will be drawn to the A7 cameras because of the less-expensive price for full frame, but will fall back on to the A7 after having looked at the A7r. For lack of a better term, this type of customer isn't as sure of what they want as the ones looking at the Df and the A7r, but they are drawn by the promise of having a relatively affordable full frame camera that is also reasonable in size and weight. In many ways, the A7 is the "serious" camera for people who don't want a DSLR but want something more than what a crop-sensor mirrorless camera can offer. To that end, it's a better all-around choice than the A7r, especially because of its faster autofocus and burst mode.
Compared to the A7, the lack of a kit lens on the A7r is also another giveaway of its particular intended target. Neither of the Sony cameras make for good mainstream professional replacements, not in native lens choices nor in ergonomics, but that doesn't mean that the A7r doesn't appeal to a sophisticated user.... a Leica user, that is. The overall speed and operation of the camera is much more measured than on the A7. The high resolution 36mp sensor and off-set microlenses make it an ideal body for accepting M-mount lenses, which a number of early adopters have done. Pairing an A7r and any Summilux lens produces exceptional results. The ring adapter for M-lenses are svelt and don't add much to the size of the camera. That said, it would be an odd-ball choice to build up an M-system from scratch using the A7r and Leica lenses; the A7r, though an interesting option, doesn't feel mature enough to make a case for replacing "the real thing," and there's also the issue of the extra mechanical tolerance issues introduced by using an adapter.
The A7r also isn't a replacement for the D800, which shares (somewhat) the same sensor. Shooters looking at the D800 aren't just after the resolution, but also the ergonomics and performance of semi-professional DSLR camera body. No doubt the A7 will pick off some DSLR users looking to shed weight, but the lack of native lens choices (at launch) is a significant deterrent.
The Df, no surprise, tends to appeal to shooters who already own DSLR's. It's more of a curiosity for younger photographers, especially those who never had a chance to use film. Even though its retro-aesthetic design introduces some practical usage challenges, it's still very much a photographer's camera. In comparison, the A7 still feels a bit like a consumer-electronic device even though it's a more focused product than ever before for Sony. The Df is a highly polarizing camera; it tends to produce love-it-or-hate-it... or sometimes... love-it-and-hate-it reactions out of people. In contrast, the A7 seems to produce interest, but of a much more clinical nature.
In the years to come, it will be interesting to see what the future holds for each camera. Even though both are full frame and technically capable, neither has its strongest case as being its technical prowess. For the Df, it is all about the emotional connection with the camera. For the A7 and the A7r, it is the value... the value of having performance in a lighter package. Neither is a particularly perfect camera, as there are shortcomings in handling and design choice in both. However, the A7 feels like a new beginning for Sony, and gives hope for what a future A8 might turn out to be. The Df, is a bit of detour for Nikon; as a smaller-run niche camera it's been a success so far, but it remains to be seen where Nikon will go with it.
With thanks to Broadway Camera