My last two cameras have been professional models with the d3s and d4. I have owned dx cameras including the d300. The d4 just lets me go and shoot images and just gets out of my way. I live in western Oregon and we have a lot of dark, cloudy and rainy weather. I need high iso's when shooting my 500mm lens sometimes with converters. The d4 runs circles around any dx camera and is Nikons best camera for shooting at high iso's. Very little post processing on most images especially for much noise reduction and frame speeds I need for my bird photography. Yes once you own a professional griped camera it would be hard to go back. I tried out the d800 by renting one for 10 days, sent it back and got the d4 and haven't looked back.
The color transition in the d4 is smooth and is much better than the d3s, the auto focus is top of the line.