To get things in perspective, an increase in pixel count of approximately 63%, comparing the D800 with the 5D3, is not huge but is definitely worth something.
If one considers an image resolution of 240ppi as being the minimum for critical viewing of a print, then the 5D3 image is good for a 24"x16" print without interpolation, and the D800 good for a 30"x20" print, approximately (actually a bit larger, like 31.5"x21").
If one needs to crop because the lens was not long enough, to get the FoV that a 7D would produce with the same lens for example, then one ends up with an 8.7mp image, hardly better than the resolution one would get from an old 20D.
If one crops a D800 image to the same degree, bearing in mind that the Nikon DX format is slightly larger than Canon's cropped format, one gets a 14.2mp image (36.3/2.56).
I never got the opportunity to upgrade my 20D to 14mp. Canon usually seems to upgrade its cameras in small increments of pixel numbers, from 6mp to 8mp to 10mp to 15mp to 18mp, with cropped format, and 11mp to 12.7mp to 16mp to 21mp to 22mp with full-frame.
Perhaps the most interesting upgrade in the 5D3 is the claimed 2 stops of better DR. If this proves to be two stops of improved DR across the entire ISO range, then that would be fantastic. It would mean that the DR at high ISOs would be on a par with the Nikon D3s, and DR at base ISO on a par with the Nikon D7000.
If the improvement in DR only applies at high ISO, as it does with the D3s, that's still a big plus, but just not as awesome.
According to DXOMark DR tests, the D7000 pixel, although smaller than the 5D2 pixel, has
over 2 stops better DR than the 5D2 pixel at base ISO. At equal print sizes, the DR of the D7000 image is
only 2 stops better.
