A lot of the success of the Sony E mount is the result Canon users tired of the Canon sensor under-performance who bought Sony bodies to mount their existing Canon lenses on.
100% of the 3 Sony a7 users I personally know belong to this category.
I am not saying that the a7x isn't a good camera, but now that Canon is picking up with better sensors, it will remain to be seen whether those guys stay with Sony (that will probably mean they like the EVF experience) or move back to Canon when image quality gets back to a higher level (which will probably mean they prefer OVFs or to use their lenses natively).
Another thing, is that our view at LL is strongly biased by the high average age of posters here who like smaller/lightweight bodies such as the Sony. I am not convinced this is representative of an overall trend in the market.
I am sure there are many, but I don't know any Nikon photographer having moved to Sony.
The main problem of Nikon, as highlighted by Thom, probably isn't the higher end FX bodies most of us are using. It is the lower end of the market where they have been losing. As part of that, they have also wasted a lot of cash in misguided initiatives such as the 1 series (although it had great technological merits), KeyMission,...
This being said, for high end users like us:
- Their recent lenses releases have been really outstanding, especially the new 70-200 f2.8 E FL, 105mm f1.4, 19mm f4 T/S,... everything their lens design team touches turns into gold it seems,
- They have all the technological blocks in their hands to do wonders also (including on sensor AF,...) when their higher mgt decides to do so,
- They have best in class image processing with Expeed, which is demonstrated by their systematic superior usage of Sony sensors compared to Sony themselves,
- They have by far the best AF system in the D5 and D500,
So there are worst situations to be in.