I now have some real shooting behind me with the D800E and thought I'd write about my experience. The camera is fairly intuitive and it hasn't taken me long to get comfortable using it. 99% of my work is on tripod and using live view. I set the camera up so a single press of the centre back button zooms to 100% (not the highest zoom that seems to be over 200+ and interpolated and somewhat pointless).
I was working in relatively low light last night and at small apertures and did find the noise on live view to be a problem. Canon does live view at wide open which reduces this tendency, and at the same time reduces depth of field so it's easier to find the sharp point, but with the risk of focus shift (of which we are hearing more and more). Some lenses are more prone to focus shift and it's hard to pedict which - largely it is worse with fast lenses, but the Sigma 35 mm. 1.4 doesn't have focus shift while the 2.8 Nikkor 14-24 reportedly does. I can live with Nikon's way to do things and it wasn't a huge effort to open up, focus and remember to close down again before the shot.
The camera stopped working properly at one point last night and nothing would fix it - turned out the battery was low, but the camera didn't indicate it was anywhere near totally out. I won't make much of this for now and it's even possible reinserting the old battery might have solved the problem - but either way something is odd. Mind you my old 5D2 used to lock up occasionally but it's a bit disheartening to see it so soon in a new camera. I have not yet updated the firmware so that could fix the problem. Will report back.
Image quality on live view is decent but def. not as good as on the 5D3 - but I can live with it. I don't like that the self timer has no beep or flash or in fact any way to know it is counting down on the back of the camera (a light flashes on the front - lot of use that is to me).
But what about the quality of the photographs - after all, that's why I switched? To be honest, it needs a pretty big print to see any difference (50 inches) and even then it isn't a do or die situation. That said, it seems to me the images hold up better to editing and perhaps don't need as much editing (early days yet). It's remarkable to see images on screen at 100% magnification that look like they aren't magnified.
I'm really impressed with the 70-200 f4 Nikkor - and I'm not convinced that switching to a series of fixed focal lengths is a better option - given how often one's position is not adjustable nearer or further from the subject - usually yes, but probably 25% of the time it isn't and I'd have to crop if I used a fixed focal length lens (85, 135, 200).
I had occasion the other night to photograph indoors with the D800E and used my 35 f1.4, wide open, ISO 2500 - and was very pleased with the results. My Panasonic GH2 with its slow 14-140 sure couldn't have handled this, and the depth of field at 1.4 worked just fine - ie. not much depth but adequate for faces.
Absolutely no regrets picking up the Sigma lens - my first non camera brand lens if you don't count my Lens Baby.