I've had my D600 for a few months now. It was a difficult decision which FF camera to get, I needed to balance a few requirements (landscape, portrait, candid and lots of low light stuff) as well as weight. I have had a few physical challenges over the last few years which means that I can no longer lug the sort of weight around that I could! I came from a DX camera with no real investment in FX glass so it was reasonable to look at the Canons too.
I have had the pleasure to shoot with many Nikon cameras, though not necessarily own them (sadly). Overall I am delighted with it. The IQ is exceptional - I cannot tell the difference between the output between this and the D800. It's low light/high ISO performance is almost D3S-esque, I doubt I could tell the difference, though I have only shot a few times with the D3S.
I was quite concerned about t'internet rumblings re poor low light focus performance and oil/dust issues on the sensor. I was more concerned when I looked at my first few images and noticed a dust bunny or two. I have used the in camera sensor clean and used a rocket blwoer and the dust bunnies have gone. Before this gets taken out of context, every DSLR I owned or used for any length of time gets these. When we first got the new Canon 5DMkIII in at work it had loads of dust on the sensor straight out of the box (in fact all of 6 them did to some degree), way more than the D600 had - it happens and takes 5 minutes to clean. So far no oil on the sensor though which is a relief - but again not difficult to clean if needed.
As for the poor low light focussing, this is not what I am experiencing. I have sometimes experienced poor AF in low contrast but good light. In ordinary contrast scenes for example using the Nikkor 50mm f1.8G and in low light, needing iso 3200 or 6400, it has focussed quickly and accurately.
I prefer the ergonomics of this camera over the D800 too. The D4 though is nicer to use, but way too expensive (and heavy) for me just now. I prefer the tactile feel of the Nikons to the Canons, though I did like the 5DMkIII in use (and was sorely tempted to jump ship too!).
I have used my D600 in the rain and snow and it has stood up to that perfectly well so far, I have also used it on the beach too and again the weather sealing is as good as expected. I like the fact it is slightly smaller and lighter than the D800 and even when shooting landscapes I don't feel as though I need any more pixels (though 36MP would still be very nice
). I don't do a lot of high speed burst shooting, but what I have done has been fine - I doubt it is anywhere near as good as the D3/D4 cameras though.
Given my weight aversion just now I will be looking at trying the new UWA Nikon have just announced and at some point trying the 70-200 F4 too.
So far so good!