I just spent a week shooting two D800's full time - one my full spectrum Astro converted D800 and the other a standard D800, both with Zeiss ZF.2 glass. This was a long exposure / night shooting trip and so these were my weapons of choice vs my IQ160 & DF/Alpa that I'd rather be using.
Bottom line for me was this - the D800 is a very very competent 35mm camera system with extreme versatility. The Zeiss glass is gorgeous (IMHO). The images were technically great with superb detail, nice colour and super resolution.
However, truthfully, I really hated going back to using 2:3 image ratio and I just didn't "gel" with the D800 at all. Maybe that's a format thing because I much preferred the D4 I got to try (I've always been a D1/D2x/D3/D3s/D3x guy ....).
On my way home I spent some time shooting with my DF/IQ160 again and it was so much nicer composing 4:3 ratio, using the DF (which I normally loathe) and Phase glass, and generally shooting with the bigger camera. Sure it doesn't have a fraction of the capabilities for long exposure, high ISO and lens flexibility of the Nikon but it was so much "nicer" to use and the images seem less 'digital' to me.
I didn't have time to shoot the Alpa this trip but even the DF outfit was so much more pleasurable to use. Normally I avoid using my DF when I can shoot with the Alpa!
These may seem very emotional terms for the shooting experience but this thread reminded me of my own journey to medium format which I can't currently contemplate changing. I'll continue to use the D800 outfit and maybe it'll grow on me more but I somehow doubt it. I bought it for specific uses and for those it is superb but my true love is still shooting with my MFDB.