Transitioning from DX back to FX I'm finding the smaller AF coverage to be one of the main differences that requires adjustments on technique.
I've been using SinglePoint AF and AF-On button on my DX bodies. With the D300s the dedicated AF-On button and Multi-Selector are first rate. Both are well positioned and the multi-selector is robust & perfectly sized. With my D5100 I found the AE/AF button to be awkward to reach, being a smaller body, the button is always behind my cheek bone and a hassle to get to.
In my experiments with the D600 I've found using the AE/AF button as AF-On to be a very important technique because of the smaller AF area. Using the AF-On button makes focusing within the AF coverage and reframing smooth (as you can release the shutter button and not have the camera refocus after reframing when re-pressing it). Luckily the AE/AF button on the D600 is reasonably easy to reach.
I'd be interested to hear how others are setting up and working with the D600 autofocus for different photography situations. How accurate and useful do you find the Auto-AF Area face recognition? With the limited AF coverage is the 3D tracking practical? What else?