Stan's reply is excellent and covers all the bases. I'm replying as an effort to balance out some of the unnecessary hysteria that followed the D600 and yes there were some legitimate issues of excess dust apparently due to a poor choice of shutter material.
I have a refurbished D600 that was purchased in January of this year. It had no dust that I could detect. One week later (after a few hundred shots), I saw a few spots. Using the in-camera dust removal and a Rocket Blower, the dust was removed. No big deal. I'm now multiple thousands of shots later and no visible dust on any of my absolutely gorgeous (in my own mind) photographs. I performed the f22, blue sky test and yes there were a lot more dust spots, but nothing has shown up on my photos. This time I'll use the Rocket Blower and a liquid cleaner, even though the dust doesn't affect my photography.
Cleaning the sensor once a month would be worth it for the level of quality it delivers. If the D600 got dropped or stolen, I would replace it without hesitation.
"Red is gray and yellow white, but we decide which is right ....and which is an illusion"