If I'm honest the price of the D600 brought me to FX, as it anabled me to trade my D300 and DX lenses. Inevitably though there was mission creep and I considered the D800, but there were no D800s in town a week ago, and the local price of the D600 was excellent and so I didn't go there.
I'm very happy with my choice as I didn't want a large DSLR body, nor 36MP, and occasionally 4 FPS might be a tad slow.
I now like the controls of the D600 just as much as the D300 etc, though I do miss that 100% crop zoom feature.
I have managed to work around the liveview/video aperture bug very well by setting aperture selection to the lens ring on my (mainly) AFD lenses.
I suppose the better AF in low light in the D800 might be missed if I had ever had it. And with 36 MP you could crop even more - so you'd have a powerful digital zoom, but I'.m not really fussed about that.
U1 and U2 modes are great as they are fixed as my base settings for people. I like that all now, though it's hard to see which one you're using in the dark.
I am really looking forward to getting the wifi going. Prefer the dual SD cards too.