My personal experience with the D600 is that there is an obvious drop-off of oil/dust on the sensor as the number of exposures increases. There were noticeable spots on some images (typically in that upper left corner that has been reported) that slowly increased as the shutter-count went up. I cleaned the sensor at about 2,000 exposures and found very little oil/dust spots after that. I cleaned the sensor again at around 3,500 exposures and have seen nothing other than the random dust one always gets from field shooting/lens changing. At this point I probably won't send my D600 in to Nikon, even if they make the offer to repair it under the heading, "If it aint broke don't fix it."