This is for all D600 owners who recently suffered from the sensor issue (dots coming from the top left corner and spreading rapidly), or who are afraid that it might come up. Here is my experience with Nikon support.
First of all, it took about 9 months for my D600 to have the issue. From the chat I had with the Nikon guy, I'd say your D600 is likely to suffer from it at some stage. If it hasn't yet, don't worry.
Is that really a pain ?
Yes, it really f***s up images and the required post processing is too heavy. On this above example, it is a very mild occurrence, it can get all over the top half of the shot and ruin it.
Is there a temporary fix?
The sensor cleaning function makes it a bit better, but you need to do it every 2 hours at least.
How is Nikon about it?
Great, they acknowledge it fully, and this issue is covered even if you are not covered by warrantee anymore.
What does it take?
I took it to my local Nikon center, I reckon you can also ship it there. They said i'd be fixed in two weeks, it was. They now replace the entire defective part with the new part -the sensor cage- from the D610 (which is no more than a fixed D600), so it is a permanent repair.
In other words, you can safely buy a D610 , and if you are tight on money, you can buy a D600 and leverage the faulty sensor cage to get a lower price (I reckon the remaining ones should come cheap). If it starts malfunctioning, Nikon will be totally cool about it.