Very pleased with mine. For what I do the ISO argument versus the D3 is fairly neutral as the 1DIV produces stonking images at 6400, and I have never needed to go above that on a floodlit sports pitch. It is well ahead of the 1DIII in this regard. The extra croppability from the higher number of pixels however is a massive bonus, and gives really useful effective range extension over the 1DIII.
Focus is better than the 1DIII. Lock on with a 300 2.8 is so fast it's hard to believe. It tracks very well.
Image quality is fab - definite improvement over the 1DIII again.
Lots of nice little features including the focus point orientation change between landscape and portrait modes.
Oh, not that I use it much, but the video is just stunning, especially with wide aperture lenses.