This was quick. DxOMark, which does scientific tests on digital cameras and ranks them based on the results of those tests, just finished testing the Nikon D600. And it scores really, really high.
In fact, the only digital camera that scores higher than the D600 is the Nikon D800, and it does so just by a hair. Every other camera DxOMark has tested, including many medium-format cameras, score lower.
The D600 is an exceptional deal at $2,100 (body only). You could pay $900 more and get the D800, but (unless you've just got money to spend) I don't really see why you would. We'll have to wait and see where the Canon D6 ranks, but my guess is it will be lower (maybe significantly lower) than the D600.
If I were in the market for a full-frame DSLR, the Nikon D600 would be the camera I'd buy. It might just be the best all-around DSLR ever made.