I finally got a great deal on trade-in gear and made the switch from 5DII to 5D Mark III ... and I gotta say, if the D800 is any better, then it must be an insanely good camera. did some shooting at 1-stop underexposed at ISO 6400 last night with the new 35mm f/2 IS and yes, while the blacks are a bit crushed (I feel this also has something to do with the lens, the 35 f/2 IS is ridiculously contrasty, maybe almost a little too much so), there is excellent detail in the areas where it matters and the files from the 5D Mark III clean up far nicer than the ones from the II in terms of noise reduction. back when I was comparison shopping, it looked like the 5D Mark III had a 2/3-stop advantage over the 5D Mark II in terms of sensor performance based on web charts (dpreview and the like), but I'm finding in real-world usage, it's functionally a 1, maybe 1-1/3 stop advantage in terms of true usability.
I haven't done enough architectural/landscape work with it yet to tell if the dynamic range is improved over the 5D II, but from the bit of work I have done so far, it looks pretty good. I've shot far more restrictive film formats before so dynamic range, limited or not, doesn't bother me. I do think more is always better, but for those folks on here clamoring that the dynamic range limitations of the Canon are a deal-breaker are definitely exaggerating. please look at the work of Galen Rowell to see how dynamic range is controlled at the point of capture. and if you claim that it's too cumbersome, remember that half the time he was photographing in locations that he either had to ski to get to, hike to get to, or be roped in to get to.