The D600 delivers very clean JPEGs up through ISO 400. You can start to see a little degradation in shadow areas at ISO 800, though there's no corresponding degradation in well-lit areas until about ISO 3200. Images are generally quite usable through ISO 1600.
While there's a lot more recovery latitude in the shadows than the highlights in the D600's images, there's sufficient latitude in both directions to fix most high-contrast photos (like my ISO 100 test photos, which I accidentally shot at +2/3 stop exposure compensation). One big difference between the D600 and the D800 is the amount of recoverable detail in seriously blown-out highlights; the latter is much better.
(1/25 sec, f7.1, +2/3 stop, ISO 100, matrix meter, AWB, standard Picture Control, 24-85mm lens at 68mm)
The clipped highlights on the background apples are an example of areas of unrecoverable highlights. This is from the 14-bit raw file shot in the Adobe RGB color space; in the JPEG, those areas are simply flat.
There are practically no visible compression artifacts or color noise in the sky, though there's some color noise in the gold areas of the clock. That tends to be difficult to process thanks to the reflection of the stoplight.
(1/80 sec, f4.5, spot meter, ISO 800, AWB, standard Picture Control, 24-85mm lens at 75mm)
Interestingly, the D600's JPEGs aren't exceptionally better than those of the 5DM2 at ISO 12800. However, the D600 doesn't have any hot pixels and has less clipping in the shadows of the raws than the 5DM2.
(1/80 sec, f4, ISO 12800, spot meter, AWB, standard Picture Control, Nikon 24-85mm lens at 24mm/Canon 24mm f1.4 lens)
The default Standard Picture Control doesn't seem to push saturation, but it does boost the contrast -- you can see where you lose some shadow detail because of it (the green peppers in the foreground, for example).
(1/60 sec, f4.5, ISO 200, matrix meter, AWB, Nikon 24-85mm lens at 80mm)