Rather than simply doing a "drive-by" trashing of the product, it would be a lot more constructive if you could provide settings and a lot more details of what worked and what didn't. Did you try different lenses, what was the environment, did AF work at all or was it just out of focus, did you upgrade the firmware to the latest rev? Do you have any photos (with the EXIF data) that show the issue so that everyone can review it? The photographers on this forum are excellent at identifying root causes, provided sufficient data is available, so provide the data and let them have a look. If there is a problem, you have a lot more data to provide Nikon to address the issue plus the resolution of the issue helps others if they encounter it.
Sorry, but when someone reports the exact same problem with 3 random samples of a product with no data to back up the statement, I get skeptical.
My personal experience with the D600 is exactly opposite from yours.
"Red is gray and yellow white, but we decide which is right ....and which is an illusion"