Like almost anything else "it depends". I shoot sports at ISO 6400 and think they are OK if I've got most of the frame, but quite limiting if heavily cropped. 3200 is much better. I've shot 12800 and they are usable as "snapshots".
But if I were doing something for real print use (i.e. that was to get near a frame) or pictures of people that someone might look closely at, or for heavy cropping, I always try to get down in the 1600 range or under, where it is VERY clean.
It also depends (at least to the tune of 2-3 stops) how much dynamic range the subject has. If your lighting is even and not over a lot of range, then you can shoot higher. If you have deep shadows and bright highlights, and it is quite dark, I try hard to keep it down under the 2000 range or so as it's there you get the best dynamic range. IT also helps in shooting higher if there are no people in it, as then you don't get that obvious plastic look in post processing.