I used to sing the high tune on DX cameras, well I still do, but now I shoot exclusively FX.
My main reasons to switch was that my good old D90 eventually succumbed the sea-water accident I had some years ago and I couldn’t get myself into buying a true replacement. D400 still not here and the D7000 feels awkward for me, most importantly I wanted the focus system of the D300 …
Forget it, the D800 does its job.
So how is FX different from DX?
Main issue DoF: Compared to DX you have to stop a FX one stop down to get the same DoF. Compare this with a base ISO of 100 compared to the 200 of the D90-300 you find yourself at four times longer shutter speeds. You need more light and/or a better technique to pull this out.
Real challenge: The D800 is a high-res body, diffraction is an issue. Shoot it f/11 or less and say good-bye to pixel-level sharpness. This creates real problems when shooting landscapes when you discover that f/5.6 at 14mm is not just “set to 1.5m and everything is sharp”. Next on my shopping list is a Tilt/Shift lens (I am eyeing on the Samyang 24 T/S) to solve this problem.