I used a pair of 5DII's from the day of launch for my professional wedding and landscape business. Previously I used a pair of 5D's. My lens list is very specific, for weddings I use either the f2.8 zoom trinity (16-35IIL/24-70L/70-200IIL) if the light is good or I run with primes if the light is low (24IIL/35L/50L/85IIL/135L) and these lenses are usually shot wide open and close to MFD to create a pleasing background blur. I used the single centre point and re-compose technique, AI one-shot and I had the fine focus screen fitted (EG-S). This technique is tricky to use but once mastered gave me faultless results. I've shot a lot of weddings in tricky light and this approach nailed the focus every time.
Now that I've migrated over to a pair of 5DIII's, I can still use that technique...in fact I'm finding those central 5 AF points are simply amazing. I'm shooting sharp and in focus images during 1st dances, which are so dark, it's all black to my eye. The camera is nailing the focus in light levels which I can't even see in and the high iso ability of this camera is nailing the exposure beautifully too.
What I gain with the 5DIII model isn't the image quality, but it's the plain and simple truth that every other aspect of this camera is massively improved over the 5DII. The build quality of the 5DIII is way way better than the 5DII or 7D. It's not far off a 1DsIII. The dual cards is a massive bonus, the 100% viewfinder is a delight to use. The built in level is great as is the 1x1 aspect crop mode. The silent mode is spookily quiet...from about 1 ft away it's effectively silent. The extra stop or so high iso ability is very nice too. My 5DIII's show practically no pattern or colour iso noise, it's remarkable.
For photojournalism / weddings there is in my opinion, the 5DIII is the best camera currently available. Bar none. Nothing else comes close, it's perfect for that role. The other great point is that it's currently the most versatile camera on the market and it nails pretty much every photographic genre in a very capable manner. I recently photographed wild red grouse who are very shy creatures. The 5DIII's AF was spectacular in AI servo.
It's fast 6fps came in very handy too.
As to the DXO marks....well, I can only guess that someone in that particualr lab is on the payroll of Nikon and their recent changes to their testing methodology now favours Nikon sensors over Canon.
The new D800 Nikon is a fantastic machine, but it is no where near the Canon 5DIII in terms of versatility and for photojournalism it's not a very good fit any more....which is bizarre considering that was the D700's strongest suit