Tried 5DIII and D800. My impression:
1. With 400mm~600mm, Gee, 5DIII is like a machine gun, super fast and super accurate.
2. On the other extremes, 17mm or 24mm, T&S. 5DIII rules again. It's unique.
3. In the middle, 50mm/1.2, 5DIII performs better than D800 but with standard zooms, 24-70 and 70-200, it's the other way.
But in this range, M9 stands out. M9 does not go beyond 135mm, but it makes 130mm ~ 200mm irrelevant.
My impressions: It's M9 for 24~135mm, and beyond these range, it'd be 5DIII. D800 is a very good body, but need lenses to match it.
I own a Nikon D800 with 24/1.4G, PC-E 24, 50/1.4G, 85/1.4G 105 f/2.8G VR and 70-200 2.8G VR lenses, I also own a Leica M9 with Summicron 35, 50 & 75 mm 6bit coded lenses. I have had the D800 since April and have done a myriad of shooting. In my experience, there is just no way that anyone can tell me the M9 has better IQ or exceeds the D800/E.
Many of the supposed comparisons that I see on the web compare cameras with lenses that are not top notch glass. This colors the results and makes them fairly insignificant.
Anyway, just my experience....