I use/used all of these cameras. I think the 5d2 is an amazing value, and frankley is better than most photographers uing them. Ellis is correct, the 5d3 is the 1ds3 replacement, but without the larger finder. The AF of the 5d3 is really VERY good, it is precise, fast, acurate. Noise performance is better, but I never had a problem with the 5d2 noise performance up to 3200. The D800e is really fantastic. Great AF, not as good a sthe 5d3, great ISO. The only problem I have with the D800 is that I'm not particularly fond of modern Nikon lenses. They are too sharp, much like the Zeiss zf and ze lenses. I shoot people, and find them too well corrected, while the cheaper Nikon lenses have too many ugly problems like vicious CA at wide apatures. That being said, I like the old AI and AIS lenses on the D800. Smooth, not overly sharp.
That being said, I don't really see much difference in a print between the 5D2, 5D3, D800e (or P30+ or Aptus 75s). I can make the D800 prints better, and there is way more head room, less noise, etc with the D800, but in reality, I shoot in controlled light, tethered, just as I did the 1ds when it first came out. So in a way, these cameras are interchangeable, with the difference being AF, lenses and viewfinders. None of the viewfinder are great. I like the Canon 50 1.2, 85 1.2, and the 135 2 so much, an dthe 5D3's AF is so good, that that has become my default camera for the studio.
Speaking of lenses, The Zeiss CF Hasselblad lenses are GREAT on all of these cameras. Focusing through the Viewfinders can be a pain, but if you can use live view the results are worth it. I stopped thinking in terms of 35mm, medium format, large format. I use these cameras for everything up to what I formerly thought of as large format portraits. Using live view, a release, and a tripod with CF lenses is exactly how I shot the Mamiya RZ and 4x5, but with no dark cloth.