I for one am very thankful for the advances in dynamic range in the current crop of bodies. I find that the transition from primarily using a 5dMKII to a new 6D has been striking in the ability to gather more detail, particularly from the shadow areas. I find that my use of HDR (which I have used in the past, particularly to expand dynamic range) is drastically reduced now as I find that I can frequently pull the detail I need out of a single RAW exposure.
I find the greatest expansion has been in the shadow area - I still see room for advancement in the highlights. A slightly underexposed image is easier to recover than an overexposed one.
I haven't used a D800, but it is the first recent Nikon product that made me wish that I wasn't so invested in Canon architecture. I have seen some pretty special images made with it. That being said, it has been my observation that Nikons frequently look better on paper than they execute in practice.