My first comment after taking my first frames with the D600 were Holy #$@*! Mind you I've been shooting the D800 since April. I was shooting indoors at ISO 3200, and the image that came up on the screen looked like my D3s but with smoother tones and better looking colors. I was floored.
To be honest, if the D400 doesn't come by Christmas, I may just get another D600.