It is by far the BEST small (35mm) format camera that I have ever used in 20+ years of working as a photographer. If that is too ambiguous let me know and I'll clear up any questions.
Eric's post matches what I've read on other pro forums. For the 5K price, this is the unit to beat and I'm a Canon guy at present. If nothing comes along from Canon soon, I'll be getting a D3 within a few months. Crying shame to have to buy another lens but the speed is essential in my work.
The D3 is an incredible improvement over its previous cameras, and I have tried the MKIII and prefer the "look" of the D3"s images. The controls are better than the MKIII's as well. I have had the D2x, D200; and also have the D300 - great camera as well.
The D3 is the best camera for what I do; a lot of low-light situations.
I agree with the above posts. I couldn't get a second one fast enough. The D3 allows for low light sports shooting. Fast focus and great viewing screen.