Haven't had the opportunity of fully "testing" the new acquisition. However, and not surprisingly, this is the best camera I'd owned, never having owned a full-frame before. Most sites that compare it to the Nikon 800 give the edge to the last one because of its higher resolution, lower price and, if I remember correctly, the dynamic range. But most of them admit better results with high ISO settings, faster autofocus and tracking and general speed with the Canon. They conclude neither is reason enough to change systems, which is a rather obvious verdict.
All I can say is that the clarity of the photos is superior. I took some interior photos of a church yesterday at ISO 1600 and 3200 and I didn't notice the noise on my computer screen. Yes, the Nikon shots are surely sharper, and I obviously hand't experienced that firsthand, but these 5D shots have a certain creamy quality (in a good way) that keeps the overall coloring and textures in a way that I just open the pictures on the screen and they look as if I don't have to do post-processing at all, except a bit of sharpening. Of course, that also depends on the lens. Also, the fast focusing is very evident. The 7D was as quick as I thought possible, and I absolutely still think it's a great camera, but lately I was using third party lenses and the focusing was much slower.
5D bad: steep price. Good for me: button layout very similar to 7D, almost no learning curve at all, except for focusing system - 17 pages, I think, in the manual, a thing I'll never probably learn to use completely, color fringing correction for registered lenses, weather sealing and very fast. Pretty much everything else is still the image strength, which doesn't depend on the camera's abilities. And I'm switching metering options, not using evaluative but center weight. For some reason, evaluative tends toward underexposure. Heavier camera bag weight now... And I don't mind at all.
Nothing like a new camera to renew desire. Shouldn't be like that, but it is. However, I still keep my position that cameras are disposable, while images aren't.
BTW, just received Annie Leibovitz "Pilgrimage". Very interesting and vey beautiful. Not a "normal" photo book but much more. History is the protagonist in this one, not photos. It's as if Ms. Leibovitz had, after not having anything more to prove as a photographer, decides to let another aspects which interest her move to the front. Intricate and mature attitudes, I think. Received it yesterday, so these are first impressions.