Been spending a lot of time on here!
I used it a long with my D800 for a wedding this weekend and the D600 performed better than expected. This picture was taken using AF-C mode = no AF assist light and no on camera flash. I had a Pocket Wizard Plus 3 triggering 2 YN560's. The D600 did exceptionally well, although it is no D800. Focus in total darkness is nowhere near as good as the D800 but it is no slouch either. To be honest, I wouldn't hesitate to use it as my primary camera.