Today I picked up my Nikon D600 that I had on pre-order and gave it a trial run around the house. My initial impressions are basically what the reviews say - a cross between a D7000 and a D800, and really not all that dissimilar from my D700.
Compared to the D700, some things are worse, others better. It loses out on top shutter speed and a couple other small things, but it gains more resolution, live view AF, excellent video and many other things.
Some quick notes:
- The Live View AF works very well for a DSLR, about the same as my Sony NEX 5n and Fuji X100.
- High ISO seems to very clean. I've only tested to ISO 3200 so far, but it looks very good.
- The D600 with a Nikkor 50mm is significantly lighter than a D90 with a Nikkor 18-200mm.
- The camera feels pro, unlike say the D90 and a D5100 I have been using lately, but not "hewn from billet metal" like the D700.
Both shots below were taken at ISO 3200.
All in all, I think this will be an excellent camera.