I'm a Canon shooter that has been waiting for a newer Canon camera.
I shoot ISO 100 in studio almost exclusively. I print big, always! Print is the final destination for all of my images. I proof images at 24"x30" or 24"x36". I am just in the process of selling my six 4x5 cameras (1 left.)
I pre-ordered the Nikon 800E when it was announced and was looking forward to it. However, I took a look at sample Canon 1DX images rezzed up and compared side-by-side to the Nikon 800.
Canon 1DX ISO 400 rezzed up was better on noise than NIKON 800 100 ISO. If the 5D3 performs as well as the 1DX - which I would expect from previous history - I am going to cancel my 800E order.
More pixels of equal quality is better. More pixels in the same sensor space, as someone mentioned, is at best a wash. In many cases the edge will go to the better quality pixels because of the increased processing power in Lightroom or PS on an i7 CPU compared to in-camera processing.
We will have to see what the dynamic range looks like. I think the new Canon's have the A/D converter on the sensor? That was supposedly Sony's key advantage in capturing with low noise and higher dynamic range. Although with controlled studio lighting, absolutute dynamic range is not as critically important, as I can control contrast with my lighting.
If I had a 5D2 would I upgrade? Maybe not. But based on previous history I probably would wind up upgrading, just because of the improvements in the Digic 5 processing, and the likely video improvements with a new generation (like the Nikon 22 level sound management in camera, etc.)
Plus the 5D2 was 3 years between upgrades. So if it fits, may as well get in early if you are going to own a camera for 4-5 years.