I use my camera in the normal manner, but I also use it like a telescope.
In other words, if I see something far away, I zoom in as close as possible using the Sigma 50-500 OS, take a bunch of photos- then I zoom in much harder in 'Nikon view' to see things as close as possible.
Call me Reginald S. Potter if you like.
I use a D300s for this purpose. The D7000 is out for me as it's missing CAM3500, but maybe the new D800 (used in DX Crop mode) would be better?
I also have a D3100 (14mp) but it seems no better than the D300.
Now, I realise that few people make huge prints and are therefore unlikely to gain much benefit from a 36 mp cam like the D800, but I think I could really use a 36mp DX body to help me identify faraway aircraft.
What improvement might I see from a D800, or even a 24mp 'D400'?