The release of the D600 has been a very interesting few days. It's hard to tell what Nikon is going to do now. With the apparent focus on FX as an entry level camera; it is not known whether Nikon is making a switch to full-frame permanently and phasing out their DX sensor or whether there will be still a semi-pro DX sensor model for those who wish to shoot action. However it seems that a lot of the wildlife/action photographers seem to be making that switch to FX; partly for the higher resolution.
As a wildlife/nature photographer, I'm not yet sold on the value of FX format yet. Certainly a bigger sensor creates the opportunity to shoot high-ISO images with relatively little grain and thus be able to shoot faster shutter speeds, however DX sensors have also increased their high-ISO capability...though not as far yet. I see grain when I crop my images at ISO 800 on my D300s, but when you look at a fully-un-cropped image; you can't tell since the image is very clear. All in all, are the benefits of a entry-level FX frame worth upgrading from a semi-pro DX frame? Is the 5.5 frames per second burst shooting speed up to the challenge of the 24 MP sensor. Will there be write delay from the buffer to the card owing that there is a massive amount of data per shutter click to transfer. A delay could cost you the "money-shot".
Will I make the switch eventually from DX to FX. Undoubtedly; however I have no reason to go FX at the moment due to my D300s and any further semi-pro or professional calibre DX frames that may come out of the Nikon factory. Will I buy the D600 just to get in the entry-level door into FX? No. Because the cons outweigh the benefits right now. The only switch that would make sense for me is to go directly from this D300s to a Nikon D4 or equivalent professional grade frame (D5, D6) when I do make the switch. The professional grade frame would provide me with the frame rates and a big enough buffer that the write speeds would not impact the shutter with delays. Professional grade frames are designed and built with speed in mind. After all, the last thing you want is to have your camera have buffer lag when you're in the midst of shooting a falcon in a terminal hunting dive for a kill. With photographers it's always the money shot and anything that detracts from it is unwanted.