I had a D600 that arrived with spots on the sensor. I also purchased the Sigma 50/1.4. But, with the crap on the sensor, I decided not to go through the process of dealing with that. I sent it back and gave up on Nikon. But the Sony sensor in the D600 is terrific. I think it's the best FF sensor money can buy. The Sony RX1 has the same sensor and that little camera is incredible at 35mm with the lens optimized for the sensor.
I now have a Canon 6D. Instead of getting the Sigma 50/1.4, I now had the option to get a very nice 50/1.2, which I just love shooting wide open. Even if the D600 was problem free, I am not sure I'd give up my 6D with the 50/1.2 L. I realize this is a Canon forum and people are going to be biased in that direction, but the DxO scores are pretty reflective of what you get with these cameras. I prefer to look at the DxO scores for low light. You'll see the Canon 6D scoring just a bit better than the 5DIII and not far behind the Sony sensor. For me, it's close enough because I prefer the Canon glass. I wouldn't have gone full frame for anything other than low light shooting without a flash and the 6D excels in focusing in very low light. Not that the D600 is bad, it's not, but the 6D is better than any camera within a few thousand dollars at low light focusing.
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