I have recently added a Nikon D600 to my kit. It is a second camera to compliment the D800E. My ultimate plan is to convert it to IR where an FX sensor and additional pixels will be quite useful.
The quick evaluation is the camera is fantastic and a worthy entry level FX camera. Most of my testing has been with landscapes. Colors are very good - about the same as the D800. High ISO performance is very good. I used it for some night photography during the Orinid shower and it had very little noise. Dynamic range is very good with great ability to recover shadows.
The big changes are in the placement of controls. It is very much like the D7000 in that regard, and actually would pair well with the D7000 for an FX/DX kit. The camera controls and body size are very similar. I still don't like the ISO control on the back rather than on top of the camera. One nice improvement is the Mode selector has a button that must be pressed to turn the dial. This eliminates the problem on the D7000 of the mode changing inadvertently. I do wish Nikon would spend a little more time to make grips compatible - we don't really need new accessories for every camera body. The common battery with the D800 and D7000 makes the D600 a nice addition.
You'll like the D600. The layout is very similar to the D7000 so you'll have an easy transition. Probably the biggest thing is the DOF is much shallower with FX. That's great for subject isolation, but harder when you need a lot of DOF.
At this point the D600 has been replaced by the Nikon D610 - essentially the same body with a slightly faster frame rate and a new shutter assembly.
There were a lot of complaints about dust/oil spots on the sensor of the D600. At this point Nikon has sent a service advisory to all owners providing free replacement - even if the camera is no longer under warranty.
I still like the camera, and if you can find a refurbished model it's a real bargain.