One of the things that set Full Frame cameras apart from any other kind of cameras are the high ISOs, much more useful than the DX cameras –which aren’t bad, but the difference is really noticeable-.
Some DX like the D3200 or the D7100 have good ISOs with a limit of approximately 1200 (the point where the noise starts being noticeable), but the D600 offers the same performance with an ISO of 3000, more than twice as much, and in some circumstances you can even exceed the ISO and obtain decent results with an ISO of 5000 or even more! (Some extreme cases can even reach 12800, but that really depends on the circumstances).
The first picture, comprised by 3 photos, was made without any kind of edition, only the usual correction (to avoid bending), but the color and light were untouched (including the noise, of course). You can click the picture to enlarge it and see the grain and noise more clearly.
Like you can appreciate, I went directly for the highest ISOs, I started at 6400, which does produce a bit of noise but still gave good results, and especially considering the picture was taken in a cloudy day, without much direct sunlight.
At 12800 ISO, the results could be appropriate for some circumstances, at 25600, however, the results are totally useless except maybe for some very specific cases, as it is the highest ISO allowed by this camera (Hi2).
Two examples more with even less light, just a single conventional light bulb in the room:
The first picture was taken on a room with very dim light, using an ISO of slightly more than and an aperture of 2.8 (that explains the depth of field), and even though the noise becomes noticeable, it’s not hard to deal with, and it’s worth mentioning that both pictures have not been edited in any way, just the usual correction to prevent bending, but other than that, no edition of any kind. So at 8000 you can take good pictures, depending on the circumstances, of course.
The second was taken in Hi1, equivalent to 12800, which is pretty high, and just like the previous one; it hasn’t been altered in any way. As you can see, it can be useful in some situations, -obviously not as much as at 8000 or 6400-, but obtaining good results with it is certainly complicated.
These pictures are very simple, but they can give you an idea of how versatile the D600 is in situations where using a high ISO is absolutely necessary or the only option to save the picture. In other words, you can use very high ISOs with this model and you will not regret it.