Went to Best Buy here in San Diego to compare the D600 vs. D800 (which I own). Take into consideration I've owned the D7k as well -- which was also compared and on display. The D600 is meatier and a little taller than the d7k, feels better in hand, but the VF is not as large as the D800's VF. I tested both D600 & d800 with the 24-85VR (BB also sells this lens separately). The D800 has marginally faster AF, and perhaps a LITTLE more accurate. Maybe not the best lens to test with, but it worked nonetheless.
Anyway, yes the D800 has larger AF array, but not that big a deal. Live view on D600 is a little different. Better? I don't know, since I NEVER use Live view on ANY camera. The video on D600 is very good, as is the D800 of course. The D600 feels better made than d7k, and slightly more flimsy than the D800. The D600 feels, looks, and handles BETTER than the 5d Mk.3 they had on display as well!! I like the mode dial on the D600 a little better than my D800, but HATE the locking crap. It just slows me down. I will say the 5.5FPS is every bit of claimed speed. It is a fast little DSLR!! And yes, the placement of the ISO button is strange....but at least you know exactly where to find it without removing eye from VF.
For high ISO, I compared both D600 & D800 at 3200 and 6400 inside the well-lit store. Granted, it wasn't RAW files but they were nearly indistinguishable -- I don't know which is better. I loved using it in the 15-20 minutes I spent. It's a very, very good offering --- even at $2,100, and they will sell like $1 steaks. Considering the 5d2 is still around $2,000 (which is WAY TOO HIGH) and the 5d3 is $3,000 (or close) this D600 is a wonderful value. I still can't believe they pulled this outta the hat. Cheers to Nikon!! Let's not forget the d800 is an EXTREME VALUE at $3,000 (and I still can't believe they pulled THAT ONE off) and our collective expectations are unrealistic regarding what the price of the D600 should be. If it had been available in April/May, or I'd known it was CERTAIN to be released -- this would have been what I bought.
I love the heck out of my D800, but I could've spent the extra grand towards the 70-200 VRII. I got a phenomenal new price on my D800 ($2,925 shipped) so I can't look back now. If I was on the fence as a new FF buyer or coming from a D700/Canon 5d/5d2 there would be no hesitation in buying the D600 as my top choice. I'm a landscape shooter myself so the D800 was a natural choice coming from the 5d2 -- not to mention my 16-35VR that will be here Monday!! Highly recommended! Don't waste time over-researching reviews because: a) it's a Nikon FF and b) very little IQ difference between this and D800. Be happy and enjoy what is becoming a more budget-friendly hobby.