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.
Images come off the sensor with very little noise at ISO 400. I can't even credit the cleanness of this shot to good noise reduction; the raw version without any NR looks almost as good.
(1/13 sec, f5.6, spot meter, ISO 400, AWB, standard Picture Control, 24-85mm lens at 62mm)
The camera and sensor have extremely intelligent noise-reduction algorithms. This is the shadow area of a relatively bright shot; there's very little NR going on anywhere else in the scene.
(1/80 sec, f6.3, spot meter, ISO 800, AWB, standard Picture Control, 24-85mm lens at 78mm)
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)
ISO 800 is as high as I'd go with assuming recoverable shadow areas -- there's some clipping in the darkest areas at 100 percent -- but it handled the significant exposure fix (raw file) very well.
(1/80 sec, f10, matrix meter, ISO 800, AWB, standard Picture Control, 24-85mm lens at 55mm)
While you can see quite a bit of color noise here, it's not noticeable in a 13x19 print or scaled down 75 percent.
(1/80 sec, f4.2, ISO 3200, spot meter, AWB, standard Picture Control, 24-85mm lens at 52mm)
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)
While the JPEGs tend to clip bright, saturated areas, the detail is still there are recoverable in raw.
(1/160 sec, f8, ISO 100, +2/3 stop, matrix meter, AWB, standard Picture Control, Nikon 24-85mm lens at 24mm/Canon 24mm f1.4 lens)