I use the DK-17M on my D700 to help with manual focus. It makes it easier for me, but the D800 viewfinder is different so my experience may not apply. I find I can focus OK with the D700 as long as the subject is static and I have reasonable light. When it gets dim I have to rely on the focus confirmation light. If I am really serious I mount my Zacuto Z-Finder on the LCD and use it like an EVF.
I primarily use the Zacuto for video with my D4 and a support system of some sort is definately in order. But I also use it for shooting stills with the D4's Silent Mode which uses Live View. In that case I shoot hand held with the Zacuto eyecup against my eye for stability. I find that approach also works well with my D700 and manual focus lenses. If I have to zoom in for precise focus, then extra stability becomes very useful since the image can get pretty shakey.
If mounted on a tripod then it works reasonably well to focus using Live view. Several advantages : 1) You are focusing the actual image as captured on the sensor, not an image reflected onto a ground glass screen by a mirror, which relies for accuracy on perfect alignment of both the focusing screen and the mirror 2) you can use the "+" button to zoom in significantly and get very accurate focus on an item. 3) you can use any part of the field of view to focus on.
Modern lenses tend to have a fairly short angle of throw for focus compared to older ones. This is because of the need for fast AF. It reduces sensitivity when making MF adjustments. Viewfinder add-ons cannot address this issue.