I had a chance to play with the D600 at the Jessops store in Oxford street and thought of sharing my thoughts.
Please bear in mind that I owned a D40X and now own a D3100, so for me this is more than an upgrade and several points I see discussed on what the D600 cannot do or does not have actually has less relevance for me and many others like me who are considering an upgrade from budget to a consumer FX. So my thoughts are just about what I felt after playing with the D600 and the 50mm F1.4G attahed to it.
It felt much heavier compared to my tiny D3100, but I felt it more sturdier, comfortable and easier to hold with a firm and secure feeling. The controls are very well laid out, but I was struggling to work them at the beginning, but after giving it a few minutes, I figured it our pretty well and it felt so much better and satisfying. The viewfinder is fantastic considering what I have on the D3100 and I can just upgrade for that. The LCD screen is so much better and bigger than my D3100. The controls and buttons were great to use and I could see myself satisfied without going through several menu items and I was excited to be able to reach out and change settings with such ease on the D600. The mode dials were great to use and although I found the mode lock on shooting modes bit weird, I didn't had any issues using that.
I did some shooting using the 50mm F1.4 lens at the store. Stores have strange and challenging lighting, but the D600 nailed it pretty well with Auto WB. The Autofocus was so much more responsive, faster and snappier compared to my D3100 that I fell in love with the D600. I was also surprised to see how effectively it was picking up particular points for focus even with objects so close to each other and focus points on camera so close to each other as well. It was like somehow it was selecting objects I wanted to focus anyway in most cases. Shots on the LCD at wide apertures looked excellent and in focus in all shots I took. I tried ISO 3200 and 6400 and was gob smacked at the quality which my D3100 can never achieve. This to me, is a reason I want to upgrade to a FX body as I often shoot indoors in challenging conditions and during night times.
What I didn't like - Only one thing. The AF points are crammed up in the middle and even my D3100 has points that are spread wider than the D600. I shoot landscape format portraits and this would require me to recompose more that I'd like to do. However, I tried a few with the saleslady and found it as not such a big issue as the AF and response was so good on the D600. But, still something I am not sure why it is designed that way.
Overall, I loved using the D600 so much that I now I am really thinking too much about it. It is still expensive for me, but I am hoping it would come down in price in few months.
I am sure there are many like me who are considering a upgrade from a DX body and who do not have much knowledge on FX and for them I can say that D600 is a treat to use. I see a lot of people talking about what is missing and why Nikon didn't implement certain things etc and see their point based on their experience and expectations. But, when I look at the D600 from my perspective - I see it as a much more capable camera than what I was expecting and it would take a me a long time to outgrow this DSLR.
For people sitting on the fence on consumer DX->FX upgrade, I think Nikon has a great camera on offer, but it is still too expensive. It may not have some of the pro qualities, but it takes great pictures nonetheless and perhaps better than many pro FX bodies. Nikon has added the D600 as a Consumer FX and this is something one should not forget while evaluating D600 against others.
I understand that many are not satisfied with too many pixel on the pro D800 (and a few other things) and were expecting D600 to be a pro alternative to D800 and their expectations were not met. But, if one looks closely, Nikon didn't come up with full pro FX in a D600 and it is pretty clear from their side and hence D600 should not be blamed for what it is not and also not intended for. Perhaps a D650 or D750 would address such needs.
but I managed. 
