D600 ISO 25600 image download
You can use it up to an A4 print. It surely isn't a useless iso, depending on the scene of course.
Not unusable, but it could be better. Sometimes, a nasty color shift shows up in the shadows (from image samples, the Canon 5D III doesn't have this problem, and retains colors well above ISO 12800).
For ISO 25600, the D600 does a good job. It's more solid at that extreme setting, IMO, than the D3/D700, and those cameras are pretty good at high ISOs already. But it still remains a last resort setting.