Check Amazon.com for all of the 1-star [worst] ratings that Nikon is getting from buyers of the D600 and D800.
I often read Amazon reviews, but you have to forget about 5-star ratings ("everything is great! I'm the next employee of the month in viral marketing!") and 1-star ("I couldn't find the power switch" or "I was too dumb to read the specs!").
So I just had a look at German Amazon 6d & d600: The few factual complaints about the 6d were the known weak specs, and about the d600 concerning the oil smeared sensors - but overall both rated very positive, because of course both are excellent cameras if you ignore the "value" and "is it worth it" aspect.