with the D600, the white light is very annoying and just too slow the way it responds. but I do use my flash red illuminator and/or I use the yongnuoa 622 AF assist beam and I thank go they are there.
this sunday I shot a wedding and was doing formals outside in almost complete darkness. even if I had the most perfect vision, I couldnt focus manually. the pictures came out amazing and thank god for that AF assist beam! almost no OOF pictures. im quite shocked!